Art. XXXVIII.—Notes on the Napier—Greenmeadows Road.
[Read before the Hawke's Bay Philosophical Institute, 10th June, 1901.]
The following rough notes on the natural history of one of our main roads are given not from any scientific value that they may have at the present time, but from a possible historic interest, as the area through which the road runs is changing rapidly from swamp and mud-flat to dry land, with a corresponding and most striking change in its living inhabitants, both plant and animal. This change is from causes both natural and artificial. The natural causes may all be summed up practically in one word—floods. From the smallest up-country freshet that just tinges the river-channel with yellow to the wild outpourings of April, 1897, each and every manifestation of the power of rain helps the spread of the dry land seaward.
It is to the work of that wild Easter-tide on this road that we may turn for a vivid illustration of reclamation, change, and renewal. Before 1897 the first section of this road that is clear of the town—that between the railway-crossing and the Tutaekuri Bridge—was bordered on each side by mudflats covered daily by the tide. Tenanted by countless crabs and estuarine shells, a feeding-ground for gulls and curlews, its only plant-growth mats of sea-grass—Zostera marina—this area looked as if its time as habitable dry land was very far off. But the great flood buried the mud-flats and their denizens deep in silt, so deep that the area rose above the influence of the tide and stretched on either side a sweep of featureless sand for the rest of the winter. With the spring, however, the salt-weed, Salicornia indica, began to creep in from the landward edges, and a small green rush-like plant sprang up in great quantities. This latter plant was Triglochin triandrum. Till this visitation of 1897 it was somewhat uncommon here. Mr. Colenso, in a delightful paper on the flora of the Napier Swamp in the old days, speaks, if I remember
rightly, of finding it only along the banks of a certain creek near Clive. Another well-known botanist told the writer to look out for it under the description of “a plant like a rush, but with a fruiting-spike like a loose-headed plantain.” This fits the plant exactly, and it was found shortly afterwards on the banks of the creek by the Petane Hotel. It must have been plentiful enough in some part of the region swept over by the flood, for upon large areas of flood silt it sprang up thickly, and is now almost as common a feature on the swamp and harbour shores as the salt-weed.
These two plants, Triglochin and Salicornia, are both natives: it seems fitting that these pioneers of the silt should be of our native flora. They are followed very closely by an introduction from Europe, the buckhorn plantain (Plantago coronopus). Like the Triglochin, it showed up but little till after this flood, but has since increased enormously, and is doing very good work here, being the first plant to take hold of the swamp that is useful from a stock-raiser's point of view.
The area raised above tide-level, and all its molluscan and crustacean life buried deep in silt, the gulls and curlews left it for better feeding-grounds. and it was not till a strong square-stemmed sedge sprang up thickly in the damper parts that it could boast of any bird-life save an occasional groundlark. But now that a few species of Mollusca (Potamopyrgus antipodarum) in great numbers, and an occasional specimen of Amphibola avellana, have worked up the channels again, and there are pools in places deep enough for eels, one may sometimes see a bittern here, mostly on the seaward side of the road, which is much the lowest and wettest portion. Towards the New Cut, where salt-weed is the largest growth, it is very barren of higher life. But, if the crabs have vanished, their place is taken in point of numbers by land-loving relatives—the common “slater” or woodlouse, an introduced species, and a smaller native species of a marbled brown and white colour, the introduced outnumbering the native by a hundred to one. The vanished shells are represented, too, by a land species—the common grey slug of our gardens. Besides slugs and slaters, spiders and a small hymenopterous insect are fairly plentiful—a poor list after the rich water-life that lies buried under them.
We can hardly think that it ever crossed the minds of the builders of this road that their embankment would in time act as a boundary between two distinct zoological provinces Yet from the Tutaekuri Bridge to the Wharerangi turn-off this road has been for some years practically the boundary between sea-birds, sea-shells, and sea-plants, and land-birds, fresh- and brackish-water shells, and land-plants: the sea
groups on the mud-flats and shallows of the Inner Harbour side; the land groups in the scrub and reeds and weed-choked channels that stretch away into the swamp on the other.
The mud-flats are the resort of black swan, gulls, terns, curlews, and stilts, the last four often within a short distance of the roadway; but the swan, except in the wildest weather, keeps far out of gunshot in the harbour shallows. In the low scrub of the opposite side blackbirds and thrushes chatter and whistle, starlings and mynahs wheel in flocks, with sparrows, linnets, and yellow-hammers. Besides these introduced species natives are fairly common; bitterns may often be seen during the season, hawks figure largely, with more rarely pukeko and weka, and, still more rarely, the swamp-crake.
It is interesting to note some of the typical land-birds simulating the habits of waders and fishers. I pulled up the other day to watch a large black-brown bird wading far out in the sluggish channel by the roadside, thinking it must be a rail, but it was only a hen blackbird; and I have watched larks busy in the same manner, thigh deep on the trailing strands of the weed that chokes the channel, picking off, I presume, the small shells and water-folk that swarm upon it.
The contrast between the molluscan life of either side is as marked as in the birds; in fact, from their slow mode of locomotion, still more so. Land-birds and sea-birds trespass to a certain extent on each other's ground, but the fresh- and brackish-water molluscs of the sluggish landward channel would perish as surely in the salt water of the seaward side as would the sea-shells of the tidal channel in the fresh water. It is the roadway, acting as a dam between the outgoing fresh and the incoming salt water, that has caused this sharp distinction between the inhabitants of these channels, a distinction which would be sharper still were it not for the culverts allowing a certain amount of salt water to mix with the fresh and make it brackish. This mixing adds to the interest of the landward channel, for brackish water has its own peculiar fauna and flora. The Mollusca of the landward channel are: Potamopyrgus antipodarum (common alike in fresh and brackish water); P. cumingiani (in Hawke's Bay found only in brackish water, but reported a fresh-water shell in some localities); and P. pupoides (found only in brackish water). The Crustacea consist of great numbers of sandhoppers (Gammarus,?), both in and about the water, with slaters (Oniscus) equally abundant on the dry land. The channels and mud-flats on the harbour side have a population that must be in or about more or less pure salt water. The Mollusca are: Potamides nigra and bicarinata, Cominella maculata and funerea, Monodonta œthiops, Amphibola avel-
lana, Tralia costellaris, Chione stutchburyi, and Mesodesma novœ-zelandiœ. The Crustacea are: Crabs, shrimps, and sandhoppers.
These sea and salt-marsh shells are being driven gradually seaward year by year by the encroaching flood deposits. Tralia costellaris, a curious little member of the family Auriculidœ, was, before 1897, very plentiful in the salt-weed on the harbour side of the Wharerangi Road. But that flood practically exterminated it from this area; it is now very scarce, and is only found on the highest banks of the sea-creeks that run up to the road. It is plentiful enough still at the Petane end of the harbour, where I have found it climbing high on the rush-bushes after rain, reminding one that some tropical species of this family have taken to an inland and forest life as true land-shells.
The flora of the seaward side of this road is interesting only from its contrast to the opposite side. Once past the shooting-butts point, the tide swirls up almost to the roadway, leaving naked on its retreat mats of sea-grass (Zostera); then these harbour shallows shrink to evernarrowing channels, which lose themselves towards the Wharerangi turn-off in silt-flats given over solely to Salicornia and Triglochin.
On the landward, or rather the swamp, side there is much more variety. We get here the typical sea-marsh flora, flourishing on the neck of comparatively dry land that divides the road from a large lagoon. The pioneers of the silt-flats nearer town, Salicornia and Triglochin, are here in abundance, with wild celery (Apium australe), Samolus littoralis, Selliera radicans, and Mimulus repens. The weed that chokes the channel is a brackish-water plant, Ruppia maritima.
Apium australe is the wild celery, so common alike on our coastal cliffs and sea-marshes. It is said that Cook's seamen used this plant as an antidote to scurvy. Samolus littoralis is of interest from being the one and only representative of the primrose family native to New Zealand. Its pale-pink flowers, which it bears in great profusion, relieves the somewhat sombre colouring of this roadside during the early months of summer. Mimulus repens, a curious little creeping ally of the snapdragon, is only to be seen at one place by the roadside, and that nearly opposite the Wharerangi turn-off; but it is very plentiful in other parts of the swamp, notably round the wetter portions of the paddocks of the North British Freezing-works.
The work of the flood of 1897 has been given as an example of natural reclamation. Turning to the artificial, it is interesting to watch the inroads made by man by means of draining
and cultivation. A few years ago an enterprising person fenced in a section of sea-marsh bordering the Wharerangi Road. It consisted of a desolate stretch of salt-weed bordered by pools, tenanted by just such a population as described from the seaward side of the road. It was then ploughed (it must have been wet work in many places for the horses), harrowed, and sown down in oats. These came up strong and green on the higher portions of the paddock, then reddening, lessened and failed altogether over the lower portions, nearly three-quarters of the area. Here the salt-weed sprung up refreshed, and the crabs returned to bore again in the sodden furrows. But in spite of apparent relapse the ploughing had acted beneficially for land-plants. Cotula repens, the “bachelor's button,” sprang up thickly in the salt-weed: then on the unbroken furrows a clover (the black medick, Medicago lupulina) got a grip, and that pioneer of the silt the buckhorn plantain. Then it was ploughed again and sown with mangold-wurzels, with about the same result as with the oats—a fair crop inland, but dwindling outward to little yellow bulbs no larger than a radish. The second ploughing strengthened the land-plants; a sward of plantain and a feather-topped grass all but ousted the salt-weed. A third ploughing was followed by maize, which still left the outer edges to the plantain and grass, but brought in with it a wonderful collection of foreign weeds—fat-hen, Prince of Wales' feather, and others—which, now the maize is cut for green fodder, have taken full possession. The some-time marsh is now a paddock, waiting only the plough to fall into the same state as the monotonous grassy levels inland.
Beyond the Wharerangi turn-off the sea-marsh fauna and flora are soon lost in paddocks, whose alien weeds and grasses are encroaching yearly upon them. It is curious to note that here and there an ill-drained portion has, in spite of cultivation, gone back to its original salt-weed; and Samolus littoralis and Selliera radicans follow the drain-sides right up to Greenmeadows Township. But these drains have quite lost their sea-marsh fauna; crabs and sand-hoppers have given place to woodlice again, and of all the shells only Potamopyrgus antipodarum has survived the change to pure fresh water. It is here with its relative P. corolla and the limneids Amphipeplea ampulla and Planorbis corinna. These last three species are emphatically denizens of fresh water. If careful notes were taken year by year of their habitats, I think they would be found to be encroaching on the sea-marsh by water, just as the snails, slugs, woodlice, weeds, and grasses are by land. At present they are down as far as the Napier Park Racecourse, on the Greenmeadows side; at Napier they are in the swamp channel opposite the
gasworks; and they are in the Tutaekuri as low down as the mouth of the New Cut.
These rough notes are taken mainly from the point of view of a lover of shells and plants; but it would seem that this area would afford a most interesting harvest to those interested in crustacean and insect life, the crustacean life of the salt and brackish water giving place to the insect-life of the fresh, and of the insects themselves the littoral giving place to the inland species. The fauna of the New Cut gives a fair illustration of this. Where this canal joins the channel just below the recreation - ground the hand-net brings up small crabs, shrimps, sand-hoppers, and a crustacean very like a woodlouse with swimming-lobes to its tail-segments. Following up the Cut with the net, the crabs soon disappear, then the water-slater; sand-hoppers and shrimps become scarce; and as one nears the Tutaekuri the larval forms of insects come up in the net—the hideous masked nymphs of a dragon-fly, and lesser relatives, the sand and horny tubes of caddis-worms, with fresh - water shells and drowned land-shells, and the seeds of many inland plants, just such a haul as one may take from a mat of watercress in one of our upland streams.
Art. XXXIX.—On the Volcanic Grits and Ash-beds in the Waitemata Series.
[Read before the Auckland Institute, 5th August, 1901.]
The object of this paper is to describe a deposit of volcanic grit which occurs in a Tertiary formation known as the “Waitemata series.” This series, of Lower Miocene age, is developed from the Auckland isthmus northwards for upwards of twenty miles, and stretches completely across the Island. The volcanic grits outcrop for the most part along the shore-line, and lie conformably between the sedimentary strata. To trace individual beds in this series is a matter of great difficulty, as these not only thin out and disappear, but are in places considerably disturbed and faulted. Fossils, moreover, occur but sparingly. The grits, however, are amongst the most distinctive beds, and in nearly all cases are fossiliferous. For these reasons they
form a valuable factor in correlating different parts of the series, and hence the importance of determining whether there are several bands of grit or only one, and whether, moreover, the material is due to air-borne or water-borne sediment. It is important also to locate as nearly as possible the position of the vents from which the material originated.
The microscopic appearance of the sections as shown in the plates will facilitate the comparison of the rocks under discussion.
Dr. Von Hochstetter, in 1859, stated that beds of volcanic ashes were interstratified with the sedimentary rocks occurring on the shores of the Auckland Harbour. Subsequently, in 1864, he made the same statements, and also alluded to the remarkable blocks of volcanic rock which occur on the Whangaparaoa Peninsula interbedded with the stratified deposits.
Twenty years later, in 1879, Mr. S. H. Cox, late Assistant Geologist, reported on the country from Auckland northwards. The whole of the Waitemata series as developed round Auckland and to the north of that city he placed as equivalents of the Pareora beds and of Lower Miocene age. In his report he says, “Above these beds (Orakei Bay beds) the Parnell grit comes in interstratified with sandstone and thin beds of sandy marl; and this grit, together with a certain quantity of volcanic ash and occasional angular stones, represents the commencement of the volcanic outburst which, while some of the ash and smaller stones were spread far and wide over the sea-bottom on which the Waitemata series was deposited, attained its greatest development near the Manukau Heads, where beds of breccia at least 700 ft. and probably more in thickness may be seen resting in direct sequence on the marls, &c., of the Waitemata series, the higher beds of this series being notable for the great abundance of volcanic material which is mixed with the sand and clay. It seems probable that the volcanic activity which must have prevailed during the latter part of the deposition of the Waitemata beds, and the consequent rapid accumulation of material on the sea-bottom, may account for the great absence of animal life during the latter part of this period.”
In 1881 the same writer, in a second report on the country north of Auckland, endeavours to show that the Parnell grit overlies conformably the Orakei Bay beds.
Two years later Mr. A. McKay, Assistant Geologist, wrote a brief account of the coast-line from Lake Takapuna northwards to the Wade, in which he conjectures that the volcanic ash-bed known as the “Parnell grit” is the southern
extension of the Takapuna ash-beds. He says, “Immediately north of the lake, where the sea-cliffs are higher and the rocks better exposed, they are easily identified as those underlying the Parnell grit. … Nearly two miles beyond the lake grey sandy marls referred to the lower beds are overlaid by a volcanic agglomerate which corresponds to the Parnell grit, differing only in the coarser material which composes it, blocks of volcanic rock more than a foot in diameter being common.”
In the following year, 1884, Professor Hutton read a paper before the Philosophical Institute of Canterbury on the age of the Orakei Bay beds, in which he reviewed most of the literature bearing on the Waitemata series. He contended—(1) That there is no evidence to show that the Orakei Bay beds are older than the Parnell grit; (2) that on the whole the evidence, both stratigraphical and palæontological, is in favour of Orakei Bay beds belonging to Pareora system (Lower Miocene).
Sir James Hector, in his progress report for 1885, briefly referred to the interstratified volcanic grits in the Waitemata series. He dissented from Mr. McKay's view that the Parnell grit is the southern extension of the Takapuna ash-beds.
In 1885 Mr. James Park, F.G.S., in a report on “The North Shore to Lake Takapuna,” wrote, “In the cliffs at the end of Cheltenham Beach occurs the volcanic breccia or grit seen on the coast north of Takapuna. Here it is almost identical with the Parnell grits. At Judge's Bay the strike is north-north-east, which would carry them under the tuffs at the North Head to the place indicated at Cheltenham Beach. These sandstones, although much disturbed in places, have a general dip to the west, and at the first point north of the lake are lying on a volcanic ash-bed or breccia bed containing many large angular fragments of scoriæ and lava, which appear so recent in character that when broken off it would be impossible to distinguish them from the basalts of Mount Eden.”
Section III.—Sketch of Geology of District.
The rocks comprising the cliffs and shores of the Auckland Harbour fall readily under four heads, and serve to indicate as many distinct periods of geological time.
The oldest rocks of the district are the Palæozoic or Maitai slates (presumably of Carboniferous age), a compact indurated sandstone which covers considerable areas in Waiheke, Motutapu, and several other islands in the Hauraki Gulf, and is continued into the Wairoa Ranges.
Following these slates, lying, indeed, unconformably on them in places (as at Motutapu), are the Waitemata beds, of
Lower Miocene age. These consist of soft or muddy sandstones and friable shales. An average section usually shows bands of sandstone and softer shales alternating with each other. The sandstones are often hard, and vary considerably in texture from fine to coarse. In a few places they are fossiliferous, though as a rule fossils are absent from the great body of strata, though wood in fragments may occur. The strata themselves vary in thickness from a few inches to several feet, the shales or sandy clays forming, on the whole, much thinner layers than the sandstones, seldom, indeed, reaching a thickness of more than a foot. They are, moreover, much softer than the sandstones, and readily crumble away when exposed to the action either of the weather or the sea.
A noticeable feature of the series is the horizontal or gently undulating position which, on the whole, the beds maintain. Here and there, however, great disturbances have taken place, resulting in the rupture and dislocation of the strata, the formation of numerous faults, and the consequent obliteration of connecting-links between individual beds.
To the third group belong Hochstetter's Quaternary beds, consisting of plastic clays and sands, occurring, for instance, along the Tamaki Creek and on the southern shores of the Manukau Harbour.
The fourth group comprises the Pleistocene lavas and tuffs which have been ejected from the numerous volcanic vents in the neighbourhood and spread over the greater portion of the isthmus. The lava, consisting entirely of basalt, varies much in texture. It is, on the whole, a hard compact rock, and is always rich in olivine. (See rock section E, Plate XXVI.)
Section IV.—Grit-beds on South Side of the Auckland Harbour.
On the eastern side of Judge's Bay there occurs a band of volcanic grit some 10 ft. in thickness dipping west at an angle of about 12°. The band consists of fine volcanic material, the fragments ranging from minute specks to particles somewhat larger than a pea. The whole is firmly united together, and forms a reef which runs about 100 yards into the harbour and is exposed at low water, its hard character enabling it to withstand the action of the waves, which have worn away the softer sandstones and shales. It lies conformably between other members of the Waitemata series, is distinctly marked off from the layers both above and below, and can be traced round Point Resolution, where it forms a long outcrop on the western side of Hobson's Bay. About three miles further east, at St. Helier's Point, and again at
Tamaki Point, a similar band appears, almost identical in texture and mineral contents with that outcropping at Parnell. (In point of fact, the only difference is that the St. Helier's Bay beds are slightly coarser at the base.) To the west it appears in St. George's Bay, dipping east at an angle of 30°, the connection between the two bands being shown in section in fig. 1. Between the two exposures the sedimentary strata are much disturbed, in places being thrown on end. There seems to be little doubt that to the same band of grit can be traced the exposures at St. George's Bay, Judge's Bay, St. Helier's Point, and Tamaki Point.
Fig. 1. C. Plan. D. Section through AB, showing connection between outcrops of volcanic grit at Judge's and St. George's Bays, known as the “Parnell grit”: 1. Volcanic grit. 2. Sandstones and shales.
Following the shores of the harbour westward, a similar band of volcanic grit is met with about a mile and a half further on below Point Acheron. The exposure occurs on the shore-line, and the band appears to dip west at an angle of 10°; but since it cannot be traced into the cliff it is impossible to estimate its thickness or be sure of its dip. Half a mile beyond this, near Point Erin, a similar band, 12 ft. in thickness, outcrops in the cliff and dips west at an angle of 17°. A few hundred yards further on, at Shelly Beach Point, it again appears, lying almost vertically between the other sedimentary strata. (See fig. 2.)
The band of grit strikes north from Shelly Beach Point and sends out a long reef into the harbour in that direction. As in the case of the Parnell grit, the strata between the outcrops are disturbed and faulted. The grit in this exposure is composed of volcanic material similar to that comprising the Parnell ash-beds.
It must be noted that both these and the Parnell ash-beds are of a fairly constant texture, and contain no large included fragments. As previously stated, the Point Acheron outcrop can only be seen as a short platform on the beach. It cannot be connected stratigraphically with the exposures at Point Erin; but the distance separating the two places is not great, and in composition the bands are identical. It seems an obvious conclusion, therefore, that the Ponsonby outcrops are connected.
Fig. 2. Section from Point Erin to Shelly Beach Point, showing connection between the two outcrops of grit: 1. Volcanic grit. 2. Sandstones and shales.
It is impossible to correlate with certainty the Parnell ash-beds and those occurring at Point Erin, because—(a) A great portion of the City of Auckland is built on the land lying between them; (b) the intervening strata are much disturbed; (c) most of the intervening country is covered with tuff from some of the Pleistocene volcanic vents. But the great similarity in the material composing all these beds; the fact of there being an undoubted connection between the Parnell and St. Helier's Point outcrops on the one hand, and on the other between the various exposures at Ponsonby; and the further fact that, whereas the distance from Judge's Bay to Tamaki Point is upwards of three miles, that from St. George's Bay to Point Acheron is considerably less—would at least point towards the very great probability of the Ponsonby outcrops being extensions of those found at Parnell:—that is, the outcrops of grit occurring along the southern shores of the Auckland Harbour are merely different exposures of the same band.
Section V.—Grit - beds on the North Side of the Auckland Harbour.
Across the harbour, just below Takapuna Point, at a distance of some two miles and a half from Judge's Bay, there occurs another exposure of volcanic grit. This formation, known as the “Cheltenham Beach beds,” consists of a band of about 12 ft. in thickness containing both coarse and fine material. At the bottom is a layer a few inches in thickness of coarse grit or fine conglomerate, the particles being from
1 in. to 2 in. in diameter and slightly waterworn. Above this the material is finer and similar to that met with on the southern side of the harbour, at Judge's Bay and Point Erin. Through the band are scattered numerous angular or sub-angular fragments ranging up to 8 in. or more in diameter. Some of these are close and compact; others, again, are vesicular and amygdaloidal, the infiltrating mineral being principally calcite.
The rocks comprising the fragments are considerably altered and weathered, the feldspar crystals being many of them kaolinised. Large crystals of augite are plainly visible in most of the included fragments.
A section from one of the fragments examined under the microscope showed a microcrystalline ground-mass consisting of a triclinic feldspar, minute augite crystals, and numerous specks of magnetite, with larger porphyritic crystals of feldspar and augite, as well as those of an altered mineral. Many of the feldspars were kaolinised, and most of them contained inclusions of magnetite and augite. They were triclinic, and comprised the varieties andesine and oligoclase. Small augite crystals were numerous, but only a few were idiomorphic. The crystalline form and mode of occurrence of the altered mineral suggested the possibility of its being altered olivine. On the whole, however, the evidence was not sufficient to pronounce definitely on the constituent. The rock is obviously an augite-andesite containing perhaps a little olivine. (See rock section D, Plate XXV.; specific gravity, 2.8.)
The first outcrop of conglomerate and grit met with is at the north end of Cheltenham Beach. The band dips east at an angle of 35°, and, running nearly horizontally for about 200 yards, suddenly disappears, as shown in fig. 3. From this onwards the strata are much disturbed and faulted, in places being thrown on end. Beyond a well-marked dome a little further north the grit again appears in a band some 8 ft. or 10 ft. thick, but the thickness cannot be accurately determined as the band does not show well in the cliff. Here it dips west at an angle of about 15°, and, running out to sea, forms a long reef. From this point northwards for a few hundred yards the cliffs give place to a low sandy flat over which the sea flowed not many years ago, but which has now become dry, the land being raised partly by the wash from the hills and partly by the sand carried in by wind and sea. When the beds (Waitemata) next appear they are dipping west at an angle of 20°. Half a mile or less further north from the outcrop of grit mentioned last another exposure of the same material occurs. Here the band strikes north and presumably dips west; but, as it only shows as a
low platform on the beach, it is difficult to be sure either of the direction or the angle of the dip.
Fig. 3. Plan of Cheltenham Beach ash-beds: 1. Band of volcanic grit. 2. Sandstones and shales (shows here (2) as dome). AB. Low sandy flat called the “Narrow Neck.” 3. Outcrop of grit further north, only showing on beach.
The two outcrops showing along the shore-line at CD and again at E are evidently portions of the same land as shown in above plan. This bed, dipping east at CD, has presumably had its dip changed by the same force which formed the dome, and appears again at E dipping west. The strata at the end of the point are truncated by a fault as shown. To the southern side of the point the dip rapidly increases until at the fault the strata become vertical.
In a paper read before the Auckland Institute in 1889 Mr. Park gave a plan of the Cheltenham Beach ash-beds. In this, however, no notice is taken of the change in the direction of dip in the two exposures, nor is any mention made of the dome. Both of these matters have an important bearing on the question under discussion. Furthermore, the northern outcrop is obtained by producing the line CD (fig. 3), whereas in point of fact it occurs considerably to the west of this position.
The outcrop at F (fig. 3) may have reached its present position owing to being horizontally displaced by a strike fault running east and west. The low swamp occurring between the higher cliffs on either side may well indicate some faulting of the strata, in consequence of which the sea has been able to effect an entrance and erode the narrow valley, now become a dry sandy flat. Additional force is lent to this theory by the fact that a line drawn across the narrow neck east and west would pass through low mud-flats and the waters of
Shoal Bay, and could be continued for upwards of two miles before reaching high ground. The following diagram will furnish a further explanation of this theory:—
Suppose CG to be the line of fault. Then if the downthrow occurred on side A, and subsequent denudation planed the whole surface level, the band would appear displaced at E and F as shown, the surface at D representing the low ground between the two outcrops. Furthermore, the amount of lateral displacement would increase and that of vertical displacement decrease with the angle of hade. A normal strike fault hading at a large angle would cause the necessary lateral displacement with a relatively small amount of throw.
It would seem, therefore, that the exposure occurring at Cheltenham Beach, marked CD (fig. 3), is undoubtedly connected with that marked E, which is almost to a certainty the same band as the one outcropping further north at F.
Mr. Park states (page 5) that the strike of the Judge's Bay bed would carry it to Cheltenham Beach. This may be so, but the bed at Judge's Bay dips west, whereas that at Cheltenham Beach dips east. Besides, the strata at Cheltenham Beach are so disturbed as to render their correlation with those occurring at Parnell a matter of extreme difficulty; indeed, no stratigraphical connection can be established between the beds on the opposite sides of the harbour. Considerable difference, moreover, is to be found in the material composing the beds at the two localities. At Parnell the deposit shows plenty of Maitai slate, contains no larger fragments and extremely few fossils; whereas at Cheltenham Beach the grit is coarser in character, in places merging into conglomerate, is distinguished by numerous blocks of andesite, and is fossiliferous throughout.
From Cheltenham Beach north the strata consist of the ordinary sandstones and shales, these being overlaid at Takapuna Beach by a stream of basaltic lava from the old crater which is now occupied by Lake Takapuna. About a mile north of this crater-lake the ash-beds again appear in the sea-cliffs, lying, as before, conformably between other beds of the
Waitemata series. At this point, however, they present the curious appearance indicated by the accompanying section (fig. 5). Portions of the volcanic grit outcrop in irregular patches and detached bands on the weathered surface of the cliff in such a way as to suggest at first sight a number of different layers. After a careful investigation of the locality, I am of opinion that only one bed is to be seen, and that the appearance of different bands has resulted from portions of the grit being squeezed in between the softer sandstones and shales. The appearance has been produced subsequent to deposit. This belief is further strengthened by the fact that in places the shales between the grit are arranged in lenticular masses, thinning out and disappearing in the course of 50 or 60 yards or even less.
Fig. 5. Appearance of cliff: A. Layers of grit apparently separated by sandstones and shales. B. Grit continuous. Sandstones and shales thinning out. C. Grit appearing in detached masses through softer strata.
The grit here presents much the same appearance as that at Parnell and Point Erin, except that in places there are coarser layers with fragments up to ¾ in. in diameter. Scattered through the ash-beds are angular and subangular volcanic fragments, some of them being upwards of 12 in. in diameter. These consist of a vesicular lava of augite-andesite showing large augite crystals and numerous amygdules of calcite and the zeolite chabazite. A section made from one of these blocks showed under the microscope a microcrystal-line ground-mass similar to that seen in the Cheltenham Beach section, except that a good deal of it was composed of an altered product—probably chlorite. As in the case of the other section, the porphyritic constituent consisted of large crystals of andesine, oligoclase, and augite, and a small amount of altered olivine. The feldspars showed broad as well as fine lamellæ, and contained numerous inclusions of augite and magnetite. In some of the crystals the curious phenomenon appeared of the direction of extinction being different in different parts of the crystal. The edges and central portions extinguished at different angles, owing to the outer zone being less basic in character. Large and small augite crystals were numerous, and many of the former showed well-marked crystalline form. Specific gravity, 2.7.
The rock, it will be seen, is similar in all essentials to that found at Takapuna. (See rock section A, Plate XXII.)
Note.—The large crystal shown in the section was the only undoubted olivine crystal present. Without this explanation the section would seem to belong to a rock rich in olivine. This is not the case.
The only description of the included fragments of rock in these deposits is that given by Mr. Park, who states that the rock composing the included fragments could not be distinguished from Mount Eden basalt. It is important to distinguish at the outset between the megascopic characteristics of the two rocks. The Mount Eden lava is a well-defined basalt, rich in olivine, whereas the rock included in the deposit of grit is an undoubted augite-andesite. In some of the fragments from Takapuna ash-beds a very few olivine crystals do occur, but they are much altered, and can only be identified by the aid of the microscope.
The rocks in the neighbourhood of the grit are much contorted and disturbed. Indeed, this disturbance seems to characterize most of the sedimentary rocks in the immediate vicinity of the volcanic grits.
The ash-beds here reach a thickness of from 10 ft. to 14 ft., and can be traced for about 100 yards north. They dip landwards, or towards the west, at an angle of 15°, and show just before disappearing in the cliff an outcrop approximately horizontal. Fine and coarse grits seem to be associated without any obvious method of arrangement, shading into one another without any distinct lines of demarcation. The particles vary from the finest material up to angular or subangular fragments 2 in. in diameter. The sandstones and shales here and there thin out and disappear, and the grit shows the irregular arrangement mentioned previously. This thinning-out of the strata is probably another indication of the pressure to which the beds have been subjected. Fossils are here relatively abundant as compared to Cheltenham Beach.
Both Mr. McKay (l.c) and Mr. Park (l.c.) assert that these ash-beds are portions of the band seen at Cheltenham Beach. But no stratigraphical proofs in support of this contention are attempted; in point of fact, none are available. The Waitemata beds are obviously shallow-water deposits. None of the strata as a rule persist for any great distance. Hence the difficulty of establishing any strati-graphical connection between outcrops separated from each other by a distance of from three to four miles. The probability is, however, that such a connection does exist.
About a mile further along the beach, at a place called Red Bluff—the headland being stained by red and brown oxide of iron, suggestive of its volcanic origin—the volcanic grits
again appear in a band about 12 ft. in thickness, and can be traced northwards for a few hundred yards. Between the exposures of grit just described and those at the Red Bluff the sedimentary strata are considerably disturbed and faulted, so much so that it is almost impossible to establish any satisfactory connection between them. It is a noteworthy fact, however, that nowhere do these volcanic ash-beds show more than one outcrop—i.e., nowhere do they outcrop as a number of bands distinctly separated by sedimentary strata.
At the Red Bluff the grits become coarser, in some places passing into conglomerates, with fragments, slightly water-worn, 2 in. to 4 in. in diameter. The coarser texture of the beds in this neighbourhood would seem to indicate greater proximity to the old seat of activity. Scattered through the beds are large volcanic blocks corresponding in size, texture, and composition to those previously described; and, as in the case of similar exposures elsewhere, these ash-beds lie conformably between the Waitemata sandstones and shales. Here, however, the direction of dip is north-east, and the angle at which the beds are inclined approximately 30°.
The two outcrops at the Red Bluff, C and D, are caused, by a fault which has parted the band and displaced one portion horizontally (see section). Beyond outcrop D the band disappears in a little bay, but reappears again a short distance further on at E, where it has been bent into a syncline showing as a fourth outcrop at F. These are the only exposures of the grit in this locality. As in the case of the Takapuna ash-beds, they are fossiliferous throughout.
That these ash-beds were laid down in water is abundantly proved by the included fossils (Bryozoa) they contain. The angular character of the whole of the material, both large and small, would point to its being deposited at no great distance from where it originally fell; whilst the fact that the bands lie conformably between the other members of the Waitemata series tends to indicate that these grits were laid down horizontally and subsequently tilted by the same forces which caused the inclination of the other sedimentary beds.
As already mentioned, these grits in several places are fossiliferous. Indeed, the fossils are abundant and can be readily seen standing out from the weathered surface. In the softer strata above and below they hardly occur at all, the gritty floor having been evidently more favourable for their growth, and the calcareous nature of the grit having aided in their preservation. Mr. S. H. Cox says that these deposits of grit represent the commencement of the volcanic outbursts which culminated in the formation of the volcanic breccia at the north Manukau Head, and further adds that this volcanic activity may account for the great absence of life during the latter part of this period. This explanation, however, does not seem satisfactory, since it is in the volcanic beds that fossils are most numerous; whilst, moreover, many of the Polyzoa and Bryozoa occur in the positions they occupied whilst living.
This brings us to a consideration of how the material reached its present position. It is hardly conceivable that it was ejected into the air and fell as the product of a single shower, for in that case one would expect to find the fossils either at the top or bottom of the beds—possibly in both places—whereas some, at any rate, of the exposures are fossiliferous throughout their entire thickness. The more probable explanation is that the material was furnished gradually, and carried along the bottom by currents working out from the sources of supply. In this way a large amount of volcanic material could be spread over the bottom with sufficient slowness to admit of the growth of marine organisms, yet rapidly enough to mark off the deposit from other members of the series.
Section VI.—Grit-beds of the Whangaparaoa Peninsula.
The next important outcrops of these volcanic grits are on the Whangaparaoa Peninsula, where they occur in considerable numbers and exhibit great variety of texture. As elsewhere, they lie here conformably between the other members of the series, which have been much disturbed in their vicinity.
Starting from the northern side and travelling east, the ash-beds are first met with in a place called Coal-mine Bay. Here the grit occurs as a well-marked band dipping north at an angle of 8°. In composition and appearance this bed corresponds with those described previously, being perhaps, on the whole, a little coarser in texture. It varies from fine to coarse, the coarser material being below, shading into a fine conglomerate. The exposure at this place is not large, but about half a mile further on the same band outcrops again and forms a reef running for some distance into the sea. The included blocks, which are numerous, consist of a hard, compact, fresh-looking andesite, with large porphyritic crystals of augite and a triclinic feldspar. Examined microscopically a section from one of these blocks showed a ground-mass and porphyritic constituents almost identical with those already described. The feldspars comprise the varieties andesine and oligoclase, and, as in the other sections, show numerous inclusions of augite and magnetite. The augite crystals are perhaps larger than those in the other rocks described, whilst the olivine, although showing crystalline form, is much altered, and is not present in sufficient quantities to form an essential constituent. Chlorite, evidently an alteration product, occurs in several places in abundance. (Specific gravity, 2.8.)
From Coal-mine Bay east the grits and fine conglomerates are numerous, cropping out at intervals all along the coast. They are similar in appearance to the one just described, except that they become distinctly coarser towards the point to the east The beds here have been so disturbed and the exposures in places are so small that it is impossible to establish any stratigraphical connection between individual outcrops. In both grits and conglomerates fossils occur from top to bottom. The included fragments here reach a diameter of from 1 ft. 6 in. to 2 ft., and occur in greater abundance than in the grits nearer Auckland.
Towards the end of the point great blocks of andesite appear, measuring in one case at least 18 ft. across. This huge mass must have fallen on the bottom and been covered up with sediment. The force with which the mass fell may have displaced the strata, causing the appearance presented in the figure in the margin. It may, moreover, have fallen in water.
But the strata round and about the block are considerably weathered, one bed shading into another without any clear line of division, hence the difficulty of explaining satisfactorily what has actually taken place.
In the case of other included masses the beds do not seem to be in any way displaced, and in no case has any alteration in the sedimentary strata been brought about at their junction with these masses. This phenomenon is not easily explained. The rocks are not in the least dyke-like in character. They have effected no alteration in the surrounding beds. They cannot, therefore, be explained as being the remains of intrusive masses. They must have reached their present position when cool, and cannot have fallen with much violence on the soft yielding sandstones and shales, which otherwise would show more signs of being displaced. Moreover, such huge masses cannot have been hurled for any great distance through the air, and the fact that they are not waterworn forbids the assumption that they have been transported far by the action of water; and yet there is no indication of any volcanic vent in their neighbourhood further than that furnished by the presence of the blocks themselves. It may have been that they were deposited quietly on a relatively hard firm bottom and subsequently covered with sediment.
At the extreme eastern point of the peninsula is a series of volcanic deposits some 16 ft. in thickness, with horizontal
outcrop, as shown in accompanying sketch. Below is a coarse conglomerate, merging into breccia, containing subangular fragments up to 3 ft. in diameter, none of the blocks being much waterworn. Above, the conglomerate gradually becomes finer, passing into coarse and then into fine grit, the whole being capped by sandstones and shales. Throughout the grit angular volcanic fragments up to 1 ft. in diameter are scattered, and the whole, as elsewhere, is fossiliferous.
The rock of which these blocks are composed is hard and compact, with very large augite crystals and crystals of feldspar showing plainly. Under the microscope the rock appeared to be identical with that found at Coal-mine Bay, except, perhaps, that the ground-mass was almost holocrystalline and the olivine more difficult to identify.
From the presence of such large masses of volcanic rock it would appear that the vent from which they were discharged was somewhere in the immediate neighbourhood, though I could find nothing to show in what direction it was likely to be discovered. Possibly its site may be further north or east beneath the waters of the Hauraki Gulf.
On the southern shores of Whangaparaoa similar bands of grit occur at intervals, but the outcrops are not so well marked as those on the northern sidé, and the material of which they consist is finer than that met with on the opposite shore. There is little doubt that these outcrops are merely the southern extension of those found on the northern side of the peninsula.
Section VII.—Grit-beds of the Manukau Harbour.
Along the northern shores of the Manukau Harbour there occur numerous outcrops of volcanic grit, both fine and coarse deposits being abundant. In several places connection can be traced between the outcrops, but more frequently this is not possible. There is, however, with one exception, to be mentioned presently, absolutely no evidence to show that the bands are distinct.
The same formation prevails here as that found along the shores of the Auckland Harbour, and in every case the grit lies conformably between the sandstones and shales of the Waitemata series. In many places the strata show evidence of having been subjected to much disturbance, and faulting is common all along the coast-line.
The first outcrop of grit appears on the beach about a quarter of a mile from the Village of Onehunga. It consists of angular fragments ranging from very fine particles to those the size of a pea, but in this place the outcrop is so weathered that it is difficult to determine its composition. It here disappears underneath the shore-line.
A few hundred yards further on a similar band outcrops, having at its base a layer of fine conglomerate (the particles being about 2 in. in diameter) some 3 ft. in thickness.
A quarter of a mile west of this spot another exposure occurs, distinctly coarser than that first met with. The whole band, about 10 ft. thick, is composed of fragments of scoriæ and volcanic ash up to ¾ in. in diameter, and contains a great deal of wood in minute pieces converted into lignite. This bed can be traced for some distance, but finally disappears in the cliff. It reappears, however, half a mile further on, maintaining the same thickness and texture. Scattered through the band at this spot are a considerable number of angular volcanic fragments, many of them being 1 ft. or more in diameter. They are imbedded in the grit, and seem to have been thrown out when that material was deposited. The rock is a hard fresh-looking andesite, with minute steam-cavities, showing microscopic crystals of augite and feldspar. (Rock section C, Plate XXIV.)
Examined microscopically an average section showed a microcrystalline ground-mass similar to that appearing in the other sections described, though containing perhaps a little more feldspar. The porphyritic constituents were identical with those in the other sections, except for the absence of olivine, no traces of which were to be found. In some of the feldspars appeared the same peculiarity as that seen in the section from the Takapuna ash-beds, successive zones of the material of the crystals extinguishing at different angles. Augite and magnetite were very plentiful, the former appearing in both large and small crystals, and showing well-marked crystalline form. (Specific gravity, 2.7.)
From this on westwards for a couple of miles there are frequent outcrops of these ash-beds. In several places they contain large angular included fragments similar to those just described. Here and there they pass into conglomerates whose particles are in some cases considerably waterworn. The conglomerates for the most part, when traced upwards, pass into finer grits. The coarser portions do not contain fossils. In some of these conglomerate beds I found pebbles of Maitai slate. These, about 1 in. in length, were flattened and much more waterworn than the volcanic material. None of this slate has yet been found nearer than Motutapu, an island in the Hauraki Gulf abous five miles north-east of Auckland. The inference is that these pebbles have either been derived from beds of slate in the immediate neighbourhood underlying the Waitemata series, or they have been brought into their present position by the agency of rivers or currents, which have carried them from more distant localities.
After passing this spot, about a mile beyond a headland called “The Horn,” the outcrops of grit disappear, and are not met with until about five miles further on, when they again become plentiful. They are similar in every respect to those already described. One exposure, however, deserves special mention. It occurs at Shag Point. As shown in sketch (fig. 9), this consists of a band of conglomerate composed of fragments of a close-grained andesite, 6 in. to 8 in. in diameter, much waterworn and rounded. This bed, 3 ft. in thickness, rests on beds of shale some 4 ft. thick; and this, again, is underlaid by a layer of grit 2 ft. in thickness. A small layer of shale separates this from another band of grit 12 ft. thick, which extends to and passes under the shore-line. The strata here dip into the hill and disappear. To the west the coast-line curves round to form a bay, whilst to the east the cliffs give place to a low gently sloping bank covered with vegetation; hence this is the only section available. Above, the vegetation extends down to the conglomerate and the rocks are completely decomposed, so that it is impossible to say what may overlie the band—most probably the ordinary Waitemata sandstones and shales. It would appear from the section that there are at least two, and possibly three, distinct bands, one of conglomerate and two of grit, separated by layers of shale. This conjecture is rendered all the more probable from the fact that the conglomerate overlies the grit. Here, however, we are approaching a country where in early Tertiary times there was great volcanic activity; hence it is not surprising to find an increase in the amount of volcanic material interstratified with the ordinary sedimentary deposits.
The country from Puponga Point northwards consists of coarse volcanic conglomerate intersected by dykes and lava-streams of andesite and beds of volcanic tuff. Masses of Waitemata beds occur in places mingled with the conglomer-
ate, as though the volcanic explosions of former times burst through and ruptured the sedimentary strata, fragments of which were eventually enclosed with the conglomerate.
The lava-streams of this district are, on the whole, very much alike. They consist of a hard compact rock slightly vesicular, showing abundant augite and feldspar crystals. The augite is plentiful, but none of it is very large. I have examined a considerable number of rock sections from these lava-flows, and have found the rocks similar—indeed, almost identical—with those already described from the fragments in the volcanic grits. They differ from these only in the absence of olivine; but, as this mineral is not an essential constituent, its presence or absence is a matter of but small importance.
Section VIII.—Summary and Conclusion.
It will be seen from the foregoing that these volcanic grits are spread over a considerable area on both sides of the Auckland isthmus, and that they invariably lie conformably between the sandstones and shales comprising the Waitemata series. It is clear, moreover, that the material, although arranged by water, has not been brought from any great distance; at least, the larger fragments could not have been carried far. A careful comparison of the sections described—and these, it must be remembered, were taken from exposures separated by considerable distances—shows that the rocks differ only in the presence or absence of olivine, and that this difference is one not between separate exposures on the same side of the isthmus, but between those on opposite sides. Moreover, this difference is so slight that it hardly deserves to be considered at all, the rocks being identical in all essential mineral contents and in specific gravity.
Stratigraphically there is no evidence to show that all the outcrops belong to one and the same band; but the close correspondence between the fragments of rock included in them, their similarity in texture, in bedding, in arrangement, and in fossil contents, and, further, the fact that in no case, except at Shag Point, is there any exposure showing more than one band, whereas in several cases a connection can be traced between separate outcrops, all furnish evidence which points towards the probability of the various exposures being connected together. There seems little doubt that this is the case with the exposures on the southern side of the Auckland Harbour. Between the ash-beds at Cheltenham Beach, Takapuna, and Red Bluff the connection has not been established; but the great similarity presented by these exposures, and the fact that the several outcrops showing at Cheltenham Beach can be correlated, as can also those ap-
pearing at Red Bluff, adds considerably to the likelihood of a single band being accountable for the whole of the exposures along the northern shores of the harbour, perhaps—though this is not so certain—as far as Whangaparaoa Peninsula. If this be the case, and there seems to be much evidence in support of the contention—that is, if a band of grit maintaining an average thickness of 10 ft. or 12 ft. persists for upwards of six miles in a northerly direction—the inference does not seem an extravagant one that the same band, which, it is to be borne in mind, shows no signs anywhere of thinning out, should extend two or three miles towards the south. This distance would bring it across the water as far as Judge's Bay, and so connect the exposures on the opposite sides of the harbour. This, however, is only a conjecture, though by no means an unreasonable one.
In a previous portion of the paper I referred to Mr. Park's statement that the strike of the grit at Judge's Bay would carry it to Cheltenham Beach, a statement which in itself is perfectly correct, but which does not meet the difficulty of correlating the two exposures. Though there is much in favour of the contention, and though it is possible to imagine conditions under which the existing outcrops could be connected, yet the presence of large masses of lava in one set of beds and their entire absence in the other is not easily accounted for. The whole matter, in short, is one whose solution is beset with considerable difficulty. It was this I wished to bring out in referring to Mr. Park's statement.
From the evidence adduced it would appear, so far as the northern side of the isthmus is concerned, that we may conclude the exposures on the southern side of the harbour to be connected; that a very strong probability exists of the Cheltenham Beach, Takapuna, and Red Bluff outcrops being connected; and that there is much to be said in favour of the theory that the Cheltenham Beach and Parnell exposures belong to the same band.
Along the shores of the Manukau Harbour the beds may not correspond to those met with on the opposite side. There are no exposures on the land between the two seas, and the distance as the crow flies from the Parnell ash-beds to those nearest to Onehunga is about six miles, hence the impossibility of connecting the two stratigraphically. But, whether connected or not, these beds are identical with those found on the opposite side, and, like them, have every appearance of belonging to the same band. The outcrop at Shag Point, near Puponga, showing the separate beds, is certainly an exception; but it must be remembered the exposure was not large, and no very definite conclusions need be based on it. It may be that the lower layer of grit is a western extension of that
found nearer Onehunga, and the interbedded shales merely lenticular masses between which the grit has been squeezed, the thinning-out being obscured by the formation of the ground and the vegetation covering it. In this case the conglomerate at the top would represent a later deposit. But the presence of conglomerate on the outskirts of a conglomerate country is not surprising.
From the similarity between the fragments included in the grits and the composition of the grits themselves it is not unreasonable to conclude that they were ejected by vents which had some connection with each other. Moreover, that there were several vents there is not the slightest doubt. It is quite inconceivable that fragments of rock such as those found at Whangaparaoa, Takapuna, Cheltenham Beach, and in the ash-beds along the northern shores of the Manukau Harbour, could have been hurled for any great distance through the air; and the fact that they are not much waterworn shows that they have not been transported far by the agency of water. Hence we may conclude that there were several centres of eruption and several showers of ashes, some coarser than others, as in several places, notably at Takapuna (see Section V., above), the coarser ash overlies the fine. Moreover, the eruptions which caused the ash must have occurred about the same time, and the showers themselves must have taken place at relatively short intervals.
All traces of the localities of these old centres of activity have long since disappeared; but the evidence furnished by the material composing the grit enables within certain limits the loci of some of them to be established. One volcanic centre undoubtedly existed at or near Whangaparaoa. Such huge fragments as those found there must certainly have been derived from a vent in the immediate vicinity of their present position. Another centre evidently lay not far from the Takapuna ash-beds, the fragments in the grits there being too large to have been derived from Whangaparaoa, a distance of about ten miles. Another vent probably had its site at or near Cheltenham Beach. But in the case of the two latter it is not possible to do more than conjecture. It may have been that a single vent situated somewhere between them furnished the material for the deposits in both places, as well as that for those on the southern shores of the harbour. That it probably existed nearer to Takapuna and Cheltenham Beach than to the southern side of the harbour is shown by the difference in texture of the material in these places, that found in the beds of the former being much coarser than the other. There is, however, nothing to indicate either the precise locality or the number of these vents. It is quite conceivable that much of the material was derived from vents in the
country now occupied by the andesitic ranges of Waitakerei. The Parnell grit may well have had its origin there, and have been brought down by rivers and currents operating at a time when the physical features of the district were altogether different. But the whole question, either of locating precisely these old vents or of estimating with certainty their probable number, is one which offers but a feeble chance of ever being satisfactorily solved.
Note.—The Waitemata series can be traced further north than Whangaparaoa, good sections showing along the cliffs at Waiwera and round the Mahurangi Harbour, where the volcanic grits again appear. North of this the sedimentary rocks change considerably in character, the numerous layers which distinguish the sandstones on the Auckland isthmus giving place to thicker bands of a more highly indurated sedimentary rock darker in colour and closer in texture. The grits at Waiwera and Mahurangi are very similar both in appearance and texture to those already described, and, like these, contain numerous angular and subangular fragments of augite-andesite, some of which reach upwards of 1 ft. in diameter. Their mode of occurrence, however, does not throw any additional light on the questions raised in this paper.
Art. XL.—Notes on some Andesite from Thames Goldfield.
[Read before the Otago Institute, 12th November, 1901.]
From the Mata Stream southward, the deeply eroded surface of the crumbling Palæozoic slaty shales, which form the basement rocks of the Hauraki Peninsula, are covered with a great pile of andesite lavas, tuffs, and breccias. From the Mata northward the coast-line is occupied by the slaty shales for a distance of eight or nine miles without interruption. Between the Mata and Waikawau Streams the slaty shales are intruded by seven massive dykes of igneous rock which are well exposed in the deep road-cuttings winding around the indentations of the rocky shore-line. The general trend of the dykes is east and west, but, so far as I could discover, they do not appear to reach the valley of the Waikawau, which runs parallel with the coast-line for some
two miles, at a distance varying from half a mile to a little over a mile; nor do they crop out on the ridge separating the Waikawau Valley from the sea. These circumstances would tend to show that the portions of the dyke-like masses now exposed in the sea-cliffs and road-cuttings are the original summits of igneous intrusions uncovered by comparatively recent marine erosion. The contact-line between the slaty shales and dykes is clearly exposed in a number of places, but in all cases the degree of alteration of the clastic rock is singularly little. In the immediate vicinity of each dyke the slaty rocks are generally bent and shattered and the joints much slickensided, as if the intrusions of the igneous mass had exerted sufficient pressure to cause local thrust accompanied by shearing and displacement, more especially along the planes of bedding. At the actual line of contact the shales are merely hardened, or sometimes brecciated for a depth of an inch or two. On many surfaces no alteration is perceptible.
In my memoir on the “Geology and Veins of the Hauraki Goldfields”* I described these rocks as hypersthene-augite-andesite, from the petrological description and name supplied by the late Professor Ulrich.† A subsequent visit to the locality convinced me that an error had arisen, probably through a misplacement or exchange of a label, and in January of this year I made a further examination of these dykes, at the same time collecting examples of each for more detailed investigation. As a result of microscopic examination in thin sections I find that these intrusive masses are composed of hornblende-andesite. There is no evidence obtainable in the field to fix the date of their eruption even approximately; but, judging from the fact that they occur as dykes penetrating the basement rock, and that no hornblende-andesite, so far as ascertained at present, is known to occur associated with the gold-bearing andesitic volcanic rocks which everywhere overlie the slaty shales, it is perhaps only reasonable to infer that they are at least older than the gold-bearing andesites. All the dykes are much decomposed, making it difficult to obtain good examples for microscopic study.
Dyke No. 1.—This forms the first rocky bluff, some 30 chains south of the mouth of Waikawau River. It shows an apparent width of about 240 yards. It is a dark-grey compact rock; feels somewhat rough to the touch. Hand samples show conspicuous crystals of feldspar and hornblende, the former up to 0.5 cm. and the latter 1 cm. long. In polarised light the base is clear and crowded with feldspar
[Footnote] * Trans. N.Z. Inst. Mining ngineers, 1897, vol. i., p. 81.
[Footnote] † l.c., p. 26.
microlites and thinly dusted with magnetite, which also occurs in occasional large irregular aggregates. The feldspars are plagioclase, translucent, often much clouded with glass inclusions; not often well developed; larger phenocrysts zoned with inclusions and not much twinned; extinction angles large, indicating a basic variety, probably labradorite. Some large plates show no twinning, but exhibit a zoned structure due apparently to a succession of isomorphous layers of growth. A little sanidine is present. The hornblende is generally altered; often shows black resorption border; interior changed to serpentinous matter showing bright polarisation colours; strongly pleochroic, changing from pale yellowish-brown to greenish-brown. Calcite abundant. With dilute hydrochloric acid brisk effervescence takes place around feldspars and hornblendes.
Dyke No. 2.—This occurs about a quarter of a mile further south. It runs east and west, and shows a width of about 300 ft. It is a compact dark greenish-grey rock. Shows crystals of feldspar and hornblende plainly to unaided eye, but not conspicuously. Under the microscope the ground-mass, or base, is very abundant, clear, and finely dusted with magnetite. The feldspars are zoned with glass inclusions; in other parts clear and fresh. The inclusions are arranged round the periphery of the crystals, following the crystallographic planes, and also as irregular aggregates along cracks. A little sanidine is present. The hornblendes are mostly altered to serpentinous matter, but do not show resorption borders. Prismatic forms common. Pleochroism: α = light-brown; β and γ = dark bluish-green.
Dyke No. 3.—This occurs about eight chains further south. Its greatest extension appears to be parallel with the coast. It throws out many ramifying branches through the slaty shales; extends along the beach for nearly 350 yards. This is a dark greenish-grey rock, closely resembling No. 2 in hand samples. Ground-mass clear; not abundant; crowded with feldspar and other mineral microlites. Feldspars mostly clear, well-twinned, showing brilliant polarisation colours; inclusions numerous. A few large idiomorphic plates present in each section. A little sanidine present. Hornblendes are all much altered; outlined with black borders; centre clear or crowded with decomposition products. Calcite and a little secondary quartz present. A section cut from a sample of this dyke, obtained near southern point of contact with slaty rocks, contains nothing very marked to distinguish it from the section described above, except the condition of the hornblendes, which is less altered, being often quite fresh.
Dyke No. 4.—This occurs about 45 yards further south. It extends along the beach for some 170 yards. A dark
greenish-grey rock crowded with crystals of feldspar and hornblende just discernible to the unaided eye, and imparting a rough impression to the touch. Base clear; not abundant, but, like No. 3 dyke, crowded with microlites and crystals of feldspar and hornblende. Feldspars fresh, but often clouded with inclusions; zonally arranged. Not much twinned. Hornblendes generally altered. Calcite and secondary quartz present. Iron not abundant, except as fine dust in base and around hornblendes.
Dyke No. 5.—This begins about 400 yards further south. It trends east and west, and shows a width of about 125 ft. This rock is much altered. Colour on surface pale bluish-grey speckled with chlorite, imparting porphyritic appearance. Ground-mass clear. In polarised light presents a finely granular appearance, due to presence of grey microlites of fairly uniform size
Feldspar phenocrysts not numerous; mostly fresh, and not much twinned. Slides contain isolated crowded aggregates of small plagioclase crystals, not much twinned, but fresh, and showing bright polarisation colours. The hornblendes are mostly altered to chlorite or replaced by magnetite. All are bordered with magnetite.
Dyke No. 6.—This occurs 330 yards south of No. 5. It has an exposure about 400 yards long. A very dense blackish-green rock, with conspicuous crystals of feldspar. When wet almost dense black. Ground-mass abundant, dark bluish-grey, in places almost black, from presence of clouds of iron-dust. Feldspars plagioclase, not abundant; occur only as large idiomorphic plates, not much twinned; fairly fresh and clear. The hornblendes are completely altered to serpentinous matter. Their original crystalline forms are sharply outlined by narrow, but very even, distinct black borders of magnetite. Parts of interior often occupied by clear matter. A little calcite and hæmatite are present.
Dyke No. 7.—This begins about 300 yards further south, and thence extends to Mata Stream It is a greenish-grey rock, much decomposed near the surface; feels rough to touch; shows crystals of feldspar and hornblende, imparting a granular appearance to rock. Base clear, but not abundant; crowded with microlites and plates of feldspar and hornblende. Feldspars fresh, but not very clear, from presence of inclusions; mostly well-twinned plagioclase. Extinction angles indicate basic variety. A little sanidine present.
Hornblende occurs both fresh and changed to chlorite and serpentinous matter; some plates show black resorption borders. Apatite present; iron fairly abundant.
In a paper read before the Australasian Association for the Advancement of Science Captain Hutton, F.R.S., describes a
similar hornblende-andesite from a dyke on the shore a little north of the mouth of Tapu Creek, which evidently refers to the dyke at the mouth of the Mata Stream.*
A very dense compact blackish-green, almost black, rock cropping out on coast between McCormick's farm and Puru Flat, about four miles and a half north of Thames. Base abundant; grey-coloured, with microlites of uniform size. Feldspars fresh, plagioclase, not much twinned. Augite fairly abundant; fresh, with brilliant polarisation colours; often twinned; one plate shows marked hour-glass structure.
This is a compact dark greenish-grey rock cut 125 ft. north of Queen of Beauty shaft at the Thames, in the north crosscut from No. 11 level, 748 ft. deep. Ground-mass very clear, dusted with magnetite. Feldspars plagioclose; fresh, but clouded with inclusions; not much twinned. Hypersthene often outlined or replaced with black dust; generally affected with decomposition. No augite detected.
This rock forms the Exchange bar, one of the best-known bars or undecomposed cores of andesite which traverse the Thames Goldfield, running parallel with and separating the main lode systems of the field. At the 748 ft. level it was found to be 60 ft. wide; in the Exchange shallow level, due north-west, about 90 ft. thick; and in the 452 ft. level of the May Queen Mine, at a point about 450 ft. north of the Queen of Beauty shaft, 300 ft. thick. Like many of these hard blue bars, it shows a marked tendency to thin out in depth.
A compact dark greenish-grey rock, cut last January a few feet from the foot-wall of the Golden Age lode in the Moanataiari Mine, at the 100 ft. level, Point Russell section. Ground-mass clear, but not abundant. Feldspars plagioclase; fresh, often clouded with microlites and glass inclusions; not much twinned; mostly basic. Hypersthene more abundant than augite; contains enclosures of glass; often altered to serpentinous products, and much dusted with magnetite.
This is a dense fine-grained rock with a vitreous or glassy lustre and conchoidal fracture. It occurs as isolated fragments and large masses weighing many hundredweights on the hills around and south of Tairua Broken Hills Mine,
[Footnote] * Hutton, Proc. Aust. Assoc. Ad. Sc., 1888, p. 7.
being evidently derived from the Tairua andesitic tuffs. The masses generally present a corroded and often ropy appearance, with a pitted surface. Under the microscope it is seen to consist of a very pale-yellow glassy ground-mass, with scattered feldspars and augite, the former occurring as narrow laths and phenocrysts, apparently representing two crops of generation. The feldspar laths are arranged with their principal axes parallel to the fluxion plane. Some binary twins do not exhibit straight extinction, and cannot be sanidine. The majority of the feldspar microliths and plates appear to be plagioclase. Augite is fairly abundant, often well formed and generally twinned. Occurs both as plates and narrow laths, which lie with their long axes parallel with the fluxion plane. Polarisation colours very brilliant. One phenocryst, showing multiple twinning, encloses two crystals of feldspar. Magnetite not very abundant.
Andesitic Glass, Omahu Hill.
This is a black semi-vitreous rock speckled with white feldspars. Lustre vitreous; feels rough. It occurs as irregular masses in the grey tuffs on the Omahu Bridle-track, about a quarter of a mile on the Thames Valley side of Odlam's gold-mining claim. Under the microscope it is seen to consist of a grey glass exhibiting wavy fluxion lines, and surrounding a few large and badly developed phenocrysts of plagioclase. Some patches of the base are partially devitrified and crowded with microliths of feldspar and augite. Augite is fairly abundant. A little magnetite is dusted throughout the base.
Art. XLI—On the Secular Movements of the New Zealand Coast-line.
[Read before the Otago Institute, 12th November, 1901.]
The solid ground is popularly considered the symbol of stability, but exact observations in the older-peopled countries of Europe have shown that, on the contrary, the crust of the earth is in a state of constant oscillation. The upheaval or depression of the land from this secular movement is so slow and gradual as to produce no appreciable difference in the physical aspect of the ground affected from year to year, and it is only after the lapse of generations, and
by means of careful measurements, that it can be proved to exist. It is only along the coast-line, where sea-level affords an unvarying base of verification, that these tranquil movements can be detected and measured.
As early as 1730 Celsius, the Swedish astronomer, had noted the gradual rise of the Scandinavian Peninsula. In 1731, in company with Linnæus, he placed a stone mark at the base of a cliff in the Island of Loeffgrund, not far from Jefle, and thirteen years afterwards was able personally to verify that the Baltic Sea had retreated 7 in., or at the rate of 4 ft. 5 in. for a century.*
That the rate of movement is not always uniform over wide regions, but differential, is shown in the case of the Baltic shores of Scandinavia. For example, at the northern extremity of the Gulf of Bothnia, at the mouth of the Tornea, the continent is emerging from the sea at the rate of 5 ft. 3 in. in a century, but by the side of the Aland Isles the rise, according to Reclus, is only at the rate of 3 ft. 3 in. in the same time. South of the isles the rate of upheaval is even slower, and further south the ground moves so slowly as to appear quite stable even in a century. This region, indeed, seems to be the pivot of the oscillation, for further south, at Scania, the most southerly part of Sweden, the land is sinking gradually, as proved by the submergence of forests and older streets of the towns of Trelleborg, Ystad, and Malmoe. It was at Scania that Linnæus, in 1749, exactly determined the position of a stone, which was found after a lapse of eighty-seven years to be 100 ft. nearer the water's edge. According to Erdmann the subsidence at Scania has now ceased, or has been exchanged for an upward movement, but it will require observations extending over another half-century to verify this conclusion.†
Celsius and his contemporaries were impressed with the view that the emergence of the land was due to the recession of the sea, the changes in the relative level of sea and land being ascribed to variations in the form of the oceanic envelope. Most of the evidence available is adverse to this conclusion, and modern geologists and physicists alike are in favour of regarding the relative changes of land and sea as due to movements of the solid land only. The mean level of the sea is now generally regarded as a constant datum, not necessarily unvarying, but varying within such infinitesimal limits as to be practically constant as a verification datum.
The principal evidences of an elevation of the land are raised beaches, sea-worn caves at present beyond the reach of
[Footnote] * “The Earth,” Reclus, p. 621.
[Footnote] † Geol. For. Stockholm Forhandl., i., p. 93.
the sea, elevated sea-ledges and terraces, human records and traditions. A subsidence of the land is more difficult to trace, as each successive sea-margin is washed away or covered over as the submergence continues. The existence of a submerged forest, of fringing coral islands, or of fiords, may be regarded as perfectly reliable evidence of subsidence. In studying the oscillations of the earth it is necessary to guard against the numerous causes of error that may arise from the unceasing struggle being waged between the land and sea. Neither the encroachment of the sea on the shore-line, which may be due to progressive erosion, nor the recession of the sea, which may be due to local accumulations of alluvial detritus, are to be accepted as evidence of subsidence without due consideration. And, since secular elevation or depression of the land is always taking place, it is obvious that an encroachment or recession of the sea, due to denudation or reclamation, may coincide with a geologic upheaval or depression. Therefore in searching for proofs of such movements the student must be on his guard against being deceived by any apparent advance or recession of the sea.
The great and varied assemblage of marine formations in New Zealand, including representatives of nearly all ages, affords ample proof that this country has been subject to many alternating upheavals and subsidences in past geological times.
In the absence of human records it is impossible to definitely or even approximately determine the direction of the present secular movements on our shores. There is abundant evidence that oscillations have taken place in comparatively recent times, but there are no data at our disposal to enable us to ascertain whether the movements which produced this evidence are still progressing in the same direction. An upward motion may be succeeded by a period of subsidence, and in the absence of a means of accurate measurement it would not be safe to generalise on the evidence of what has taken place in some past time, however recent.
The buried and submerged pine forests in the Thames Valley and Bay of Plenty prove that the movement in those regions has been downward up till a very recent date. The extent of this area of subsidence cannot be defined, but the geological evidence clearly indicates that it extended as far west as the shores of Auckland Harbour. Whether this secular subsidence of the Hauraki Gulf is still in progress cannot be determined at present.
A submerged forest on the sea-shore near Waitotara, with the trunks of the trees still standing erect in the sea, points to a very recent subsidence of the land in that region. The
well-known raised beach around Wellington Harbour was upheaved suddenly during an earthquake about forty years ago, and must not be confused with the evidences of slow secular movement.
The raised terraces on the coast-line of Canterbury and Otago, and the recent excavation of the narrow rocky gorges of the Clutha, Taieri, and other rivers draining the east side of the “great divide,” point to a slow but continuous elevation of the land which may still be in progress. On the other hand, there can be little doubt that the fiords or sounds of south-west Otago were narrow mountain-glens excavated by subaerial agencies at a time when the land stood at a higher level than at present. The subsidence which has been in progress in that region since post-Tertiary times has allowed the sea to run up and fill the submerged glens. Thus each fiord will mark the site of a submerged valley.
New Zealand, from its insular position, its division by the sea into islands, its numerous harbours and extensive coastline, is destined to become an important maritime nation. It will always be dependent on the sea for its communications and commerce, both internal and foreign, and this will necessitate the erection and maintenance of harbours, docks, and coast-protection works of a costly and permanent character, specially adapted to accommodate the trading-vessels of the future. It is quite certain that, in the design and erection of these works, the direction of the secular movements of the land will be factors demanding serious consideration. Hence it is now our duty—a duty we owe to posterity—to erect around our shores permanent marks or stones, the positions of which have been accurately determined, for the guidance of the engineers of the future. For example, an accurately determined progressive and uniform upheaval of the floor of a harbour at the rate of, say, 6 ft. in a century would necessitate the introduction of important modifications in the design of dock-accommodation intended to be of a permanent character. Further, it is well known that subsidence in an area allows the accumulation of silts, sand, and gravels in the estuaries, harbours, or rivers in that area; while, conversely, the rising of an area permits the sweeping-away and scouring-out of old accumulations of alluvial detritus in harbours and similar situations. Since the engineer is called upon to combat, or at least direct, the forces of nature, he should be provided with a full knowledge of the direction of these forces, otherwise his best-devised schemes may soon become useless, if not actually destructive.
Stone marks have long since been erected on their shores by most of the civilised maritime states of Europe and America, and the time has arrived when this should be done
here; and not only in New Zealand, but on the shores of the Commonwealth of Australia. It has been shown that the secular movements are so quiet and slow as to produce no appreciable alteration from day to day or year to year. They often require a lapse of several generations to be capable of proof by careful measurement; hence the sooner the marks are erected the earlier will the data be available in the future.
The proofs of upheaval and subsidence are sometimes obtainable over wide continental areas, but generally are marked by a local and variable character; hence, marks should be erected on the shores of all our harbours, on the headlands and outlying projections of land. The work has a high scientific and economic importance, and would naturally fall to the State Department of Lands and Surveys. It could, perhaps, be most conveniently carried on simultaneously with the magnetic survey of the colony now in progress. The marking of the coasts of Australia should be undertaken by the Federal Government, so as to obtain uniformity in the method of determining a mean sea-level datum. Up till now no serious attempt has yet been made to determine the relation of sea-level to the land in New Zealand on a scientific basis, and for this reason the marking of the coast-line with stones, whose position has been accurately determined with respect to sea-level, would further supply a much-needed datum of verification for the officers of the Lands and Survey Department for their more exact geodetic and hydrographical surveys.
Art. XLII.—Notes on some Glacier Moraines in the Leith Valley, Dunedin.
[Read before the Otago Institute, 12th November, 1901.]
The glaciers of New Zealand are reputed to be the largest in existence outside the polar regions, with perhaps the exception of some in the higher Himalayas. They are found clinging to both flanks of the “main divide” of the South Island, their greatest development being within the Province of Canterbury. On the West Coast they descend to within 750 ft. of sea-level, into the midst of the evergreen forest. On the east side, where the slope is more gradual and the
annual precipitation less, the terminal face of the glacier ice is seldom found below 2,500 ft.
An examination of the present physical features of the country affords abundant evidence that the present glaciers are but mere remnants of ice-masses that once covered hundreds of square miles, in many places reaching even to the sea.
In Nelson we have the great tumbled moraines blocking the lower ends of Lakes Rotoroa and Rotoiti; in Canterbury, the marvellous ice-cut terraces on the mountains north of Lake Ohau, and the well-preserved terminal moraines on the plains south of the same lake; and, in Otago, the strikingly beautiful rounded or billowy ice-worn foot-hills of the Matukituki, in the Upper Wanaka, the gigantic moraine blocking the old outlet of Lake Wakatipu at Kingston, and the great ice-shorn plateau of Central Otago, through which the Taieri has cut its narrow tortuous course.
Naturally enough, the most abundant and most obvious evidences of former glaciation are to be found in the vicinity of the present-day glaciers, on the ground the glaciers have passed over twice, once in advancing and once in retreating.
On the other hand, there is nothing to show that New Zealand ever experienced a glacial period corresponding to the Ice Age of the Northern Hemisphere. The evidences of glacier action just mentioned show clearly enough that ice-masses of huge size must have occupied a very large portion of southern Otago and Southland in Pliocene or Pleistocene times; and it seems equally clear that an extension of the present glaciers seaward would explain the origin and source of these ice-masses.
Up to the present time no traces of glaciation have been found in the North Island. The continuous and widespread series of older and newer Pliocene strata in the Wanganui, Wèllington, and Hawke's Bay districts, with their rich assemblage of marine forms, proves the existence of long-continued sedimentation in shallow seas, teeming with life, at a period when the great ice-plough was scooping out the valleys of Otago. The circumstance that probably 98 per cent. of this varied fauna is represented by living forms shows that the climate in Pliocene times was neither warmer nor colder than at present.
I do not propose to minutely discuss the causes which led to the refrigeration necessary to permit the great extension of the glaciers of Otago in Pliocene times. This subject has already been exhaustively dealt with by Sir James Hector, Captain Hutton, and others. It will be sufficient to state that the former believed, as the result of his explorations among the West Coast Sounds of Otago in 1863, that an
elevation of 2,000 ft., together with the greater extent of land then existing, but since removed by ice erosion, would be sufficient to extend the glaciers to their former limits.
I will now proceed to describe some morainic mounds which I discovered last June in the Leith Valley, near the foot of Waikari. These moraines are situated near the top of the ridge separating the Leith Valley and Ross's Creek, which flows into the city reservoir. They extend from the saddle between the Leith Valley and Ross's Creek southward, running parallel with the Leith Valley and terminating in Mr. Henry Skey's farm, Section No. 74, at a point nearly due north of the upper reservoir. Although near the top of the ridge, they lie on the fall into the Leith. There are two lines of mounds parallel to each other. The upper mound, about 500 ft. above the sea, begins inside Mr. Skey's boundary, crosses the road, and runs in a west-north-west to east-south-east direction for about 5 chains, gradually increasing in height towards the saddle until it suddenly ends in a pile of andesitic rocks. At a point about half its length it is 8 ft. high, presenting a steep face to the south and a long gentle slope to the Leith Valley. Numerous tree-stumps and clumps of native forest around the saddle indicate that the whole ridge was at one time covered with forest. At the present time a rimu (Dacrydium cupressinum), 4 ft. in circumference and 50 ft. high, is growing in the depression of the lower mound. The lower mound resembles a line of massive earthwork constructed for defensive purposes. It is cut through by a road, and is seen to be composed, at that point, of fragments of rotten rock now forming compact yellow clays. The composition of this mound is also seen in a recent cutting, 10 ft. long and 4 ft. deep, near its western end and on its lower slope, which exposes a confused mass of andesitic rocks and clay. The rocks are mostly angular, tabular in form, and often of enormous size. The pile of tumbled rocks at the western end of the upper morainic mound was evidently exposed by the removal of the associated clays by recent denudation.
The high-level terraces in the valley of the Leith and the alluvia west of the saddle would tend to show that the Leith, before the excavation of the present rocky gorge, flowed across Ross's Saddle to the back of Maori Hill, and thence westward in the direction of the present Kaikorai Valley. The present deep narrow valley of the Leith, with its precipitous rocky sides, was obviously excavated in comparatively recent times.
Explanation of Plates XXVII.-XXVIII.
View of southern end of lower moraine looking north-west.
View taken in depression between the upper and lower mounds, looking along the depression in a west-north-west direction.
Art. XLIII.—On the Septarian Boulders of Moeraki, Otago.
[Read before the Otago Institute, 12th November, 1901.]
Plates XXIX - XXXV.
In the year 1848 Mr. Walter Mantell, as Government Commissioner for the Settlement of Native Land Claims in the South Island, travelled on foot from Kaiapoi to the southern settlements of Otago. An interesting summary of his notes by the way was published by his father, Gideon Algernon Mantell, in the “Proceedings of the Geological Society” for the year 1850 (vol. vi., p. 319).* On page 320 in that publication is a sketch-map of the geology of the coast-line so far as could be gathered from his daily observations and from information obtained from the natives, who at that time were fairly numerous. Names of rivers and coastal features are given from Kaiapoi to the Molyneux River. On reaching what he calls Onekakara Bay he gives a sketch taken a little south of Hampden, looking to the north, including the coastal features as far as the “White Bluff,” and in the foreground is “a group of septarian boulders, called by the whalers ‘The Ninepins.’”
Mr. Mantell says, “Midway between the bluff and Moeraki the clay contains layers of septaria varying from 1 ft. to 5 ft. and more in diameter. Hundreds of these nodules, which had been washed out of the undermined clay cliffs by the encroachment of the sea, were scattered along the beach. Some were subglobular, others spherical, many were entire, whilst others were broken and glittered with yellow
[Footnote] * “Notice of the Remains of the Dinornis and other Birds, and of Forest and Rock Specimens recently collected by Mr. Walter Mantell in the Middle Island of New Zealand, with Additional Notes on the Northern Island, by Gideon Algernon Mantell, Esq., LL.D., F.R.S., G.S., &c. With Note on Fossiliferous Deposits in the Middle Island of New Zealand, by Professor E. Forbes, F.R.S., &c.”
and brown crystals of calcareous spar, with which all the hollows of the septaria were lined or filled. Some of these masses were hollowed out by the action of the waves into regular basins, which at lowtide stand up from the sands full of water, and are 3 ft. or 4 ft. deep.”
He then notices the zones or belt of cone-in-cone clay with which they were encircled, and gives diagrams and sections. The diagram is interesting as showing a fragment of bone, not in the centre, as a nucleus, but close to the outer edge. The septarium is a small one in this instance, being 2 ft. in diameter, and the piece of bone enclosed ran straight into the mass. The fragment of bone was flattened, 1½ in. in longest diameter; “its cancellated structure appears to resemble that of the moa.” A note appended by Dr. Mantell (see below) states that slices prepared for examination under the microscope showed the bone to be avian. It would be of great interest to have the fragment re-examined, as neither Dr. Mantell, Mr. Tomes, nor Mr. Bowerbank do more than state that the microscopical characters show that the bone belonged to a bird. Dr. Mantell was not aware at that time of the occurrence of reptilian bones in nodules and boulders at Katiki, the Amuri Bluff, and in similar beds in other places. The probabilities are, I think, more in favour of its being reptilian than moa, more especially as no moa-bone has since been found in beds of equal age. The fragment is, I believe, still in the British Museum.
Since their discovery the remarkable character of these septaria has attracted successive generations of visitors, and nearly all the smaller specimens have been removed to adorn(?) the corners of paths and the grottoes of the suburban villas of the ingenious. Many years ago an attempt was made by the late Mr. J. T. Thomson to manufacture cement from them, a proceeding probably suggested by the analysis given by Dr. Mantell, and made at the Museum of Practical Geology in London, showing 66.7 per cent. of carbonate of lime. Dr. Mantell also compared them with the septaria extracted from the London clay on the coast of Sussex, which are made into first-class Roman cement. They occur in abundance in the Isle of Sheppey, and are dredged for off Harwich and in Chichester Harbour.
Judging from the sketch given by Mr. Mantell, the septaria were at the time of his visit well exposed. At the present time the sand appears to shift very frequently, and at times nearly covers the boulders. My photographs (see plates) were taken at lowtide, and show that at present about half of each stone is visible. There is one well down between tide-marks which is much broken on the landward
side, and has in its hollow a most charming natural aquarium, the sides and bottom of which are thickly lined with Sabellids, sponges, and beautiful sea-anemones (see Plate XXXII.). Mr. Mantell, with the feelings of a weary, footsore, pedestrian explorer, exclaims, “What an excellent footbath!”—a suggestion practical if not poetical. There is one very large specimen still imbedded in the clay cliffs about 50 ft. above tide-mark, but I could not see any of the smaller sizes. The majority of those between tide-marks are encircled with a thick coating of the small blue-black Mytilus, and the bare portion is partly covered with a vivid-green Alga, which contrasts with the mussels. Those above tide - limits are weathered to a greyish-brown, and in some cases the hard calcite filling the septæ, or cracks, stands out in relief, being less easily acted upon by the weather. Four or five large specimens have fallen to pieces and show the interior to be a hard blue clay or limestone, and numerous exterior layers, which show more and more the cone-in-cone structure as they approach the surface.
The Maori has localised the tradition of the loss of the celebrated canoe “Arai-te-Uru” in the neighbourhood by pointing out the long reef just south of Port Moeraki as the canoe, and the cargo may still be seen strewn on the beaches, a huge elongated concretion being the hinaki, or eel-basket, of Hape-ki-taurake and the slave Puketapu.* The globular septarian boulders are the calabashes which held the supply of water for those in the canoe, and a number of strangely shaped ferruginous concretions which occur to the south of the headland at Katiki are the kumaras washed ashore from the wreck. Mr. Mantell speaks of these “kumaras” as “nodules containing a far larger amount of iron and less lime than those before mentioned. The spot is known to the whalers as ‘Vulcan’s Foundry.’”
These Katiki Beach boulders are also much in request for garden ornamentation. The natural red-brown colour is often improved(?) by a coat of white paint or white-wash.
Mr. Shortland visited this part of the coast in the early days and notices the story of the canoe and the kumaras, but does not seem to have seen the larger groups of septaria to the north.†
From the geological or stratigraphical point of view the boulders have been frequently referred to in the Reports of the Geological Survey. In 1862 Sir James Hector indicated the position of the Moeraki septaria beds in a paper on the
[Footnote] * Canon Stack: “Traditional History of the Southern Maoris” (Trans. N.Z. Inst., x., 61).
[Footnote] † Shortland. “Southern Districts of New Zealand,” p. 190.
geology of the Manuherikia Valley, and also in 1864, when he determined the succession of the beds in the Shag Point district. Sir Julius von Haast reported on the Shag Point coalfield in the Geological Reports for 1873–74, page 25, and Captain Hutton, describing the Waipara formation in the geology of Otago, refers to the septaria of Moeraki. Another colonial geologist, Mr. Cox, in his report of 1877, refers to their stratigraphical position in connection with the Shag Point series of beds. Still later Mr. McKay examined the geology of the coast-line from Moeraki Peninsula to Kakanui,* and states that “overlying the lower greensands are some dark muds or carbonaceous clays more than 100 ft. thick, which in these lower beds contain the celebrated Moeraki boulders, which, whether spheroidal or flattened, are usually enveloped in a coating of cone-in-cone limestone.”
Flattened boulders covered with cone-in-cone limestone are seen in the bed of the Little Kini Creek, opposite Hampden Railway-station. The beds containing boulders reach the beach at the mouth of the creek; but there they are mostly flattened, and it is only when seen further south within tide-marks that they take the perfectly spheroidal form. There are also spherical and elliptical ferruginous concretions on the Katiki Beach, on which a few saurian remains have been found.
In the report for 1890–91, page 47, Mr. McKay has a humorous and lucid description of the difference between the grey and the brown “boulders.”
Sir James Hector also refers to them in his appendix to the same report (page 173). Captain Hutton, in his “Sketch of the Geology of New Zealand,”† includes the beds in his Pareora system.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Carbonate of lime||66.7|
|Peroxide of iron||4.7|
Contained when received 2 per cent. of water.
Note on the Fragment of Bone referred to above.
The external form of this fragment conveys no idea of its nature; but slices carefully prepared for the microscope
[Footnote] * Rep. of Geol. Survey, 1886–87, p. 223.
[Footnote] † Quart. Journ. Geol. Soc., May, 1885.
present, under a moderately magnifying power, a structure which shows that the bone belonged to a bird. There is, however, no proof that it can be referred to the Dinornis. Mr. Tomes and Mr. Bowerbank, who have obliged me by examining the specimen, concur in this opinion. Insignificant as this fact may appear, still, in these early pages of the palæontological history of our antipodean colonies, it is worthy of remark that the first-discovered fossil relic of the terrestrial Vertebrata in the Tertiary strata of New Zealand should belong to that class which, in later periods, constituted the principal types of the warm-blooded animals of the fauna of that country, to the almost entire exclusion of the Mammalia.—G. A. M.
Description of Plates XXIX.-XXXV.
Moeraki Beach, with septaria.
Art. XLIV.—Note on an Artesian Well at Aramoho.
[Read before the Wellington Philosophical Society, 6th August, 1901.]
About three miles above Wanganui, on the Wanganui River bank, a 4 in. bore has been made by Mr. John Walker, jun., in search of water. There is, of course, much speculation as to where the water comes from. It is struck under a layer of papa (280 ft. thick), and water was struck at bottom of this layer, at 540 ft. below the surface, in a layer of sand intermixed with pumice sand. The surface here may be 30 ft. to 40 ft. above the sea. Perhaps the water gets under the main papa stratum where it has been pierced by the volcanic heights about Ruapehu and Tongariro and follows down under the papa formation to where found. I found the temperature of the water coming out of the pipe at the surface to be 70½° Fahr., while the adjoining river-water was 42° at 6 ft. under the surface. The temperature in the shade at the time was 45°.
A few years ago a 2 in. bore was put down at the same
place and water was got at the same depth, and this has continued to supply houses in the neighbourhood successfully.
The new bore is 11 ft. off the old bore, and does not seem to affect it. This bore is about 1 chain off the river-bank on the right bank of the river.
Before this a trial bore, 14 chains down the river and 4 chains off the bank, was put down by Mr. Walker to 610 ft., but no successful water-supply was got, and no solid papa stratum was reported there.
The following notes were given to me by Mr. Walker:—
From surface to 60 ft., sand and pumice.
60 ft. to 66 ft., sand.
66 ft. to 105 ft., clay.
105 ft. to 117 ft., sand (wood at 110 ft.).
117 ft. to 127 ft., shingle.
127 ft. to 130 ft., sand.
130 ft. to 138 ft., shingle.
138 ft. to 144 ft., coarse grit.
144 ft. to 183 ft., sand.
183 ft. to 183½ ft., hard seam (taking five hours to bore).
183½ ft. to 185 ft., fine sand.
185 ft. to 239 ft., papa.
239 ft. to 260 ft., sand.
260 ft. to 540 ft., papa.
At 540 ft. water came up strongly, after boring through a hard seam. The water was got in sand intermixed with fine pumice, into which the rods were put down to 553 ft. The bore is piped with 313¼ ft. of 4 in. pipe, the top of last length being 15 in. below the surface (the rest of the bore has no pipe). A strong volume of water was got, with a good pressure.
Art. XLV.—The Volcanic Beds of the Waitemata Series.
[Read before the Auckland Institute, 24th February, 1902.]
The Waitemata series is a group of strata developed round the shores of the Waitemata Harbour, from which it derives its name. The upper limit of the series is well defined, since a complete unconformability exists between its shales and
sandstones and the overlying tuffs and lava-streams, which are of Pliocene or later age. The lower limit is not so certain. In this paper it is taken to be the Papakura limestone, which crops out along the Palæozoic ranges to the east of the Waitemata Harbour, lying unconformably on the upturned and denuded edges of ancient slates and phyllites. This limestone is considered by Captain Hutton to be Oligocene in age, while the beds in the vicinity of Auckland are classed as Lower Miocene. If this classification be accepted the volcanic beds with which this paper deals are Lower Miocene, some of them Oligocene perhaps.
Above the limestone lies a thick group of greensands. These are succeeded by sandstones and shales, evidently deposited in somewhat shallow water, for ripple-marks and current bedding may frequently be observed in them. There is some doubt as to whether an unconformability exists between the greensands and the overlying beds, Captain Hutton and Mr. S. H. Cox, F.G.S., both holding that there is one,* while Mr. James Park, F.G.S., believes the evidence to point to a regular succession.† At least the unconformability cannot be very great if we judge by fossil evidence.
About the time when the limestone was being formed to the east of the Waitemata there rose through the Oligocene sea to the westward a long line of volcanic vents, now denuded and overgrown with dense forest, and known as the Waitakerei Range. There is no means of ascertaining its exact extent or the nature and position of the vents. These Waitakerei outbursts gave rise to a thick bed of coarse volcanic fragments. As the bed occurs typically at Cheltenham, it may be called the Cheltenham breccia. This is the oldest of the volcanic beds of the series.
Twenty or thirty miles to the eastward another line of vents became active at nearly the same time, on Coromandel Peninsula. The Coromandel eruptions, however, would seem to have commenced rather earlier than those at the Waitakerei, and to have continued for some time after the western vents had become quiescent. The débris from the Coromandel volcanoes was spread over the floor of the sea, and possibly it was these eruptions which supplied the material for the Parnell grit, the youngest volcanic bed of the series.
Between these two main beds, the Cheltenham breccia and the Parnell grit, there are other but less important beds of volcanic origin which seem to have been all derived from the Waitakerei outbursts. Each of these seems to be less coarse
[Footnote] * Trans. N.Z. Inst., 1884.
[Footnote] † Trans. N.Z. Inst., 1889.
than the preceding one; and they mark, I believe, the gradual dying-out of volcanic activity along the Waitakerei Range.
The general evidence for the age of the beds is of a threefold character. In the first place, there are the fossils contained in the beds themselves—the palæontological evidence. This alone would show that the Cheltenham breccia cannot be Pliocene, as the Geological Survey contends. In the second place, there is the composition of the contained lava. In New Zealand, as in other places, there seems to be frequently a regular succession. The earlier eruptions are sometimes basic. These, however, are succeeded by acid lavas, and these again by more basic, the lava of a “petrographical province” growing more and more basic till the cycle is completed. The lavas of Pliocene age in Auckland are olivine basalts. The lavas of the volcanic beds of the Waitemata series are pyroxene-andesites; and, since the Eocene lavas are generally rhyolites, the evidence seems at least favourable to a Miocene or Oligocene age for the Waitemata lavas. The exceptions to this rule make it impossible, however, to rely on the lithological evidence alone—or, indeed, to lay much stress upon it. The last line of evidence is the position of the beds—the stratigraphical evidence. At Orakei Bay there is a very fossiliferous greensand of whose Miocene age there can be little doubt. It will be shown that the volcanic beds are all below the greensand, with the exception of the Parnell grit, which is above it.
Generally the beds of the Waitemata series are either horizontal or dipping in long gentle anticlines and synclines; but in places they are distorted and dislocated, and may even be thrust over each other. It has been generally supposed that these strains and the numerous faults are due to the volcanic forces which produced the basalt puys, scattered in scores round the Waitemata. Sometimes, perhaps generally, the distortions occur in proximity to a puy. At other times no such apparent connection is visible, and the strata may be disturbed far from any basalt cone, or may he horizontally quite close to one. It may be that the small puys are no true measure of the magnitude of the volcanic forces that formed them, and that, as in Scotland, when denudation lays bare the underlying rocks great sills will be found whose contents never reached the surface.*
In working out the stratigraphy of the Waitemata series the volcanic beds are invaluable. They are widespread, and their lithological characteristics are much more distinct than those of the sandstones and shales. Moreover, some of the
[Footnote] * Sir A. Geikie: “Ancient Volcanoes of Great Britain,” vol. i., p. 458.
tuffs are comparatively rich in fossils, whereas the sandstones and shales are generally lacking in organic remains.
2. Previous Observers.
Hochstetter was the first to describe the Waitemata series, in a lecture delivered to the Auckland Institute in 1859: “The horizontal beds of sandstone and marl which form the cliffs of the Waitemata and extend in a northerly direction to Kawau belong to a newer Tertiary formation, and, instead of coal, contain only layers of lignite. A characteristic feature of this Auckland Tertiary formation is the existence of beds of volcanic ashes, which are here and there interstratified with the ordinary Tertiary layers.”*
Captain Hutton showed in 1870 that the beds could be followed in an easterly direction to the Hunua and Wairoa Ranges, composed of Palæozoic slates. He thought that the estuarine sandstones forming the upper part of the series were separated by an unconformability from the greensands and limestone to the east.†
Mr. S. H. Cox, of the Geological Survey, traced the beds to the north some ten years later. At Komiti Peninsula he found Lower Miocene fossils associated with the forms found in the Orakei Bay greensand. He therefore concluded that the Waitemata series was Lower Miocene.‡
Sir James Hector, Director of the Geological Survey, thought that the series should be divided at the Parnell grit; the beds below this he classed as Cretaceo-tertiary, those above it as Lower Miocene. This seems to have been the first occasion when the importance of the Parnell grit as a stratigraphical guide was realised.‡
Mr. McKay, of the Geological Survey, examined the district in 1883. He agreed with Sir James Hector in the division of the series; considered the “Fort Britomart” shales the equivalent of the Orakei Bay greensand, and showed the Parnell grit lying unconformably on the former; and identified the Cheltenham breccia with the Parnell grit on stratigraphical grounds.§
Captain Hutton, in 1884, showed that there was no evidence of an unconformability between the Orakei Bay green-sand and the Parnell grit; that there was no evidence that the latter was younger than the former; and that the Orakei Bay bed was of Miocene age.¶
Mr. James Park, of the Geological Survey, made an
[Footnote] * “Reise der ‘Novara’: Geology,” i., p. 34.
[Footnote] † Trans. N.Z. Inst., vol xvii., p. 307.
[Footnote] ‡ Geological Reports, 1879–80.
[Footnote] ‡ Geological Reports, 1879–80.
[Footnote] § Geological Reports, 1883–84.
[Footnote] ¶ Trans. N.Z. Inst.
examination of the district in 1885. He collected a great deal of new evidence as to the stratigraphical position of the Parnell grit, which he was inclined to think inferior to the Orakei greensand; identified the Parnell grit with the Cheltenham breccia on palæontological grounds; traced the whole series eastward to the Papakura limestone; and concluded that the Waitemata series was unconformable to the Cretaceotertiary beds. His final classification was as follows:—
“Fort Britomart” shales.
Parnell grit and Waitakerei breccias.
Orakei Bay greensand.
Cape Rodney grits.*
Sir James Hector still thought the series should be divided at the Parnell grit. He dissented from Mr. Park's view that the Cheltenham breccia was the northerly extension of the Parnell grit, considering the Cheltenham breccia and the other volcanic beds to the north of the harbour of Pliocene age, and quite unconformable to the Waitemata series.†
Mr. Park, in 1889, upheld his views as given above.‡
Since 1889 nothing of importance has been published on the volcanic beds of the Waitemata series. The view adopted in the present paper has been already indicated in the introduction.
3. The Volcanic Beds of TheSeries.
As the volcanic beds at Cheltenham and Parnell are the thickest, most fossiliferous, characteristic, and widely spread beds of the series, and their stratigraphical position and age have been a subject of much dispute, most of the paper will be devoted to a consideration of them. The evidence tends to show that they are distinct beds, though Mr. McKay and Mr. Park consider them identical, and Mr. Park writes that he has “conclusively proved” their identity. Sir James Hector considers the Cheltenham breccia to be Pliocene, so that it will be necessary to give in some detail reasons for supposing it to be Lower Miocene, or even Oligocene. With regard to its source, I will give evidence tending to show that it probably came from the Waitakerei vents. The chief point
[Footnote] * Geological Reports, 1885.
[Footnote] † Geological Reports, 1885–86.
[Footnote] ‡ Trans. N.Z. Inst., 1889.
of interest with regard to the Parnell grit is whether it is above or below the Orakei Bay greensand, because the latter is a fossiliferous bed which is allowed to be Miocene. Mr. Park felt inclined to place it below, although admitting the evidence inconclusive, and he has since classed it as Eocene.* But a large amount of new stratigraphical evidence will be given which leads me to think that it is really above the Orakei greensand, and therefore Miocene. Its source is an open question; it may have come from the Waitakerei, but there is evidence in favour of its having come from the Coromandel vents. There are at least two other volcanic beds. One of these really consists of a group of tuffs separated by thin layers of shale. They are well developed at Wairau Creek, and so I have called them the “Wairau tuffs.” The other bed is a feldspathic tuff, developed best at Ponsonby, and called throughout the “Ponsonby tuff.” These can be dealt with more shortly.
I consider that the thick volcanic breccias on the west of the Waitakerei Range should really be included in the Waitemata series, but I have not been able to examine them sufficiently to include a discussion of them in this paper.
4. Are The Cheltenham Breccia And The Parnell Grit Distinct Beds?
The study of the volcanic outcrops at these places has led me to the conclusion that these beds are distinct. The evidence is considerable, consisting of a number of facts which are cumulative. Before giving them it will be advisable to give the evidence in favour of the identity of the beds.
Mr. Park, in 1885, came to the conclusion that the outcrop at Cheltenham, on the north side of the harbour, was simply a northerly extension of the outcrop at Parnell, on the south side. Sir James Hector dissented from this, and wrote: “The Parnell grit, which has been much relied on in discussions concerning the Waitemata formation, has in many cases been confounded with the volcanic grits and conglomerates in other parts of the district” †¼—i.e., with the Cheltenham breccia. Mr. McKay, too, who had been the first to suppose the beds identical, wrore: “As a consequence of my admission that the Parnell grit does or should pass under the Fort Britomart and Calliope Dock beds, and of the observed fact that the breccias north of Cheltenham Beach overlie them, I am forced to agree with Sir James Hector that the Parnell grit and Cheltenham breccia do not
[Footnote] * “Thames Goldfields”: Park and Rutley, 1897.
[Footnote] † Geological Reports, 1885–86.
occupy the same horizon, and that the Parnell grit is the older.”*
This led Mr. Park to defend his views.† To Mr. McKay he replied that the Cheltenham breccia might quite possibly underlie the Calliope Dock beds, because a basalt cone of more recent date lay between the two and obscured their stratigraphical relations. This was, so to speak, negative evidence; but he added that at Parnell, in the lower 2 ft. of the grit, he was fortunate enough to find some fossils (a Pecten, a Cerithium, a Teredo. and several small corals), and he wrote: “The Cerithium, Pecten, and corals are the same as those found in the breccia at Cheltenham, thus proving conclusively that the Parnell grit is the southern extension of that stratum, deposited at the same time and under the same geological conditions.” Mr. Park made good use of his fossils. As he does not give even the generic names of the corals they need not be considered. Surely it is possible for two beds of nearly the same age to have Pecten polymorphoides and an unknown Cerithium associated together, especially as Pecten polymorphoides has a wide vertical range and is a common Miocene fossil.
As far as I know this is all the evidence in favour of the beds being identical. Since, however, the beds are both volcanic breccias and similar, and there is a stretch of water two miles in width between the two, it is reasonable to suppose them identical unless there is good reason for thinking them distinct. The evidence for the latter view is as follows:—
(1.) The beds are not entirely similar in lithological characters. In the case of the Cheltenham breccia the bed consists almost entirely of volcanic fragments, some of them (though this is rare) 8 in. in diameter. In the Parnell grit most of the fragments are greensand, slate, &c., while only in the lower layers do we find volcanic fragments of any size, and these are rounded, well-worn scoria, generally oxidized, and never more than 1 in. in diameter. There are other minor differences, but the difference in texture is the point which I wish to emphasize: in the one bed numerous lumps the size of an orange, hard, black, and angular; in the other, red, rounded, scoria fragments not larger than marbles. This difference is brought prominently before one when trying to obtain a suitable fragment at Parnell from which to make a microscopic section. It must be remembered that the Parnell outcrop is not more than two miles and a half from the Cheltenham outcrop. It is in that distance that the texture
[Footnote] * Geological Reports, 1888–89.
[Footnote] † Trans. N.Z. Inst., 1889.
alters so remarkably. This line of argument is immensely strengthened by some outcrops hitherto unrecorded. The most interesting of these are some reefs in the harbour which are uncovered at lowtide and may then be examined. I cannot do better than represent these various outcrops diagramatically. The positions are shown by an X and Y-respectively, the Y standing for the bed with coarse texture and the X for the bed with fine:—
Now, it can be shown that very probably the coarse bed is derived from the Waitakerei vents, and it is difficult to see how such a distribution as that shown above could be effected. How is it that none of the large blocks reached the spots marked with an X, while great numbers reached those marked with a Y? Supposing this due to currents, those currents must have been somewhat peculiar ones. But this argument from variation of texture does not stand alone; it is supported by much stronger ones.
(2.) The fossil contents of the beds are different. I have examined more than half a dozen outcrops of the coarse bed at widely separated localities, and more than a dozen outcrops of the finer bed at spots equally far apart. In every one of the former fossils are plentiful, especially Bryozoa and Pectens, the total number of fossils in the coarse outcrops amounting to more than forty species. In more than twice as many outcrops of the latter I found scarcely any fossils, a species of Bryozoa (Fasciculipora ramosa) being the only usual one. Moreover, the fossils of the coarse bed* show a blending of Miocene and Oligocene forms just as we find in the Papakura limestone, while the fine bed can be shown on independent evidence to be decidedly Miocene.
(3.) The stratigraphy gives direct evidence in favour of the beds being distinct. I cannot mention all the minor facts which receive an explanation on this hypothesis and become difficulties on any other. I shall merely mention the two clearest indications that the beds are distinct.
[Footnote] * List on page 468.
(a.) At Lake Takapuna (Wairau Creek), about 30 ft. above the outcrop there of the coarse bed, are the blackbanded Wairau tuffs. At St. John's College black-banded tuffs, apparently the same, occur some 70 ft. below the fine bed. Neither section is completely exposed, but I could see no sign of a break.
(b.) At the Manukau Harbour, where the occurrence of these beds does not seem to have been hitherto observed, two outcrops occur, a quarter of a mile apart. One is a coarse bed with fossils, ten species of the Bryozoa being also found at Cheltenham, as well as Pecten burnetti and Rhynchonella nigricans. The other is a fine-grained bed, in every way resembling the bed at Parnell, except that its fragments are slightly smaller and its thickness rather less. The coarse-grained bed has larger fragments than at Cheltenham, which is natural enough if the Waitakerei vents were the source; so that the difference in texture between the two beds is accentuated (Plate XXXVIII., fig. 1).
The section cannot be seen in a direct line since two small bays occur at x and y; but there is no break possible except at one point, the head of the inlet at y, where there has been a fall of débris, so that no section can be seen. This slip may mark a line of fault, but as the beds have the same dip on both sides of it and are similar, and there is no sign of distortion of the strata, there seems no reason to suppose one. The coarse bed is here about 30 ft. thick, the other perhaps 12 ft.
I wrote that the evidence was cumulative—i.e., the stratigraphy and the palæontology both point to two beds. In fact, it seems to me improbable that so many fossils should be preserved invariably in the coarse bed and never in the fine; that the texture should vary so rapidly; and that the stratigraphical relations given above, sometimes fairly clear, sometimes obscure, but always indicated, should be always misleading.
Accepting, then, the conclusion that the beds are two distinct formations of different date, it will be well to consider them separately; and, first, the Cheltenham breccia, because it is the older.
5. The Cheltenham Breccia.
The Cheltenham breccia presents usually a bedded appearance, due to the arrangement of fragments of approximately the same size in roughly parallel bands. The coarsest band is usually about a third of the way from the bottom, and the angular fragments scattered through this band are as large as apples. These fragments are imbedded in a matrix of smaller débris, which forms the rest
of the bed. Next to this main band the lowest layers are the coarsest, those above shading off very gradually into a tufaceous sandstone. From these facts it seems probable that the main outburst took place some little while after the vents had become active, and was succeeded by outbursts less and less powerful. The thickness of the bed varies, of course, with its distance from the source: the thickness seems to average about 25 ft. at ten or twelve miles from the Waitakerei Range When we consider the thickness (3 ft.) of the ash which the violent outburst of Tarawera produced in 1886,* we cannot but be impressed with the magnitude of the eruptions necessary to lay down this coarse and thick breccia. The fossils contained in the bed are generally found either at the top or bottom.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
The bed weathers to a black or brown colour when exposed to the air, but in the finer parts is bluish-grey on a fresh fracture. The surface is very irregular, owing to the fact that the lava fragments weather out of the matrix. Near vegetation—along the top of a cliff, for instance—all the colour is generally leached out, the result being a creamy loam; or, if there has been much oxidation, a bright-red stratum forms a band along the summit of the cliff. Zeolite veins running through and through the bed are not uncommon. These veins are not more than 1/10 in. in width, but extend for yards, and when the rock is weathered they sometimes stand out on the face of the cliff like a network of miniature dykes.
The material of which the bed is composed consists almost entirely of rounded fragments of lava set in a matrix of finer volcanic débris. Sometimes the fragments are rough and angular. Occasionally blocks of sandstone or shale are included, and these are sometimes several feet in diameter. Some small fragments of porcellanite also occur.
Besides these constituents there are very numerous crystals of feldspar and augite, sometimes broken, but often retaining very perfect crystalline shape. These, no doubt, were separated from the lava in which they were contained at the time of the explosions. The feldspars are bright glassy forms in little oblong crystals, showing good cleavage. The augite crystals are of two sizes. The smaller and less perfectly formed resemble those in the lava; but occasionally much larger forms may be found up to 1 in. in length, distinguished from the former both by their larger size and more perfect crystalline form. Large crystals, and especially large augite crystals, appear to be frequently observed in tuffs deposited at no great distance from a vent. Their origin is obscure, and
[Footnote] * “Eruption of Tarawera,” Professor A. P. W. Thomas.
Sir A. Geikie remarks*: “The conditions under which such well-shaped idiomorphic crystals were formed were probably different from those that governed the cooling and consolidation of ordinary lavas.” These crystals, however, were found at a greater distance from the vents than is usual, since they must be at least ten miles in a direct line from the Waitakerei Range.
The typical lava of the Cheltenham breccia is an andesite, usually an augite-andesite, but hypersthene-andesite is also present, as is the case at the Coromandel, a parallel line of activity of much the same age. There is occasionally a tendency to ophitic structure, but it is never pronounced, and the ground-mass is typically hyalopilitic. The specific gravity varies from 2.5 to 2.8, but in the majority of cases does not exceed 2.7. This is the more remarkable since a few of the rocks are basalts (but without olivine). This somewhat low specific gravity may be accounted for, however, by several considerations. In the first place, there is frequently a fair amount of dark-brown glass in the ground-mass. Teall gives the specific gravity of andesites as ranging from 2.54 in a glassy to 2.79 in a crystalline state.† In the second place, some of the specimens are highly amygdaloidal, the amygdules forming a large percentage of the rock. They generally consist of chabasite, which has a specific gravity ranging from 2.06 to 2 17, so that the specific gravity of the whole rock fragment would be much lowered. On the whole, then, they may be taken as typical andesites, while a small percentage are basalts without olivine. Except in one doubtful case I have seen no olivine; but a highly basic serpentine, containing 0.47 per cent. of nickel, has been described as occurring at Manukau North Head, so that olivine-basalts may yet be found in the lavas if not in the ejected fragments.
Besides the more basic fragments there are others presenting the appearance of true trachytes, pale-grey in colour, with a specific gravity of 2.54. Occasionally fragments of acid pumice are also present. It is possible that this did not come from the Waitakerei vents. At the Tamaki Gulf, a few miles from Auckland, there are pumice beds, which I consider to be of Pliocene age. They lie unconformably on the Waitemata series, and are due, I believe, to the fact that the Waikato River then flowed into the Auckland sea.‡ Before this, however, it flowed into the sea near Tauranga, in the Bay of Plenty, and it cannot be supposed that it was flowing into the Auckland sea so long ago as Miocene times. Except
[Footnote] * “Ancient Volcanoes of Great Britain,” vol. i., p. 62.
[Footnote] † “British Petrography.”
[Footnote] ‡ Trans. N.Z. Inst.
the Waikato, I see no source from which pumice is likely to have been derived by transport, and it does not occur in the beds below or above the breccia. If the pumice is really part of the ejected matter, we have a large range of lava from basalts to rhyolites, indicating probably a long period of volcanic activity. In the present state of our knowledge, or lack of knowledge, of the Waitakerei lavas it is scarcely safe to generalise, but it is interesting to note that no rhyolites have yet been found in situ at the range, though andesites are abundant and andesitic basalts still more common. It is possible that the acid lavas rose in the vents but never flowed out as lava. Dykes and necks are frequently more acid than the lava-flows, and this seems here to be the case.*
I have drawn some sections of the lava as seen under the microscope, but the drawings are merely diagrammatic (Plates XXXIX. and XL.). The shaded portion represents the ground-mass (which is usually opaque, but sometimes consists of a brown glass). Owing to this fact the phenocrysts appear in the diagrams to stand out from the ground-mass more than they do in the rock. The shading, moreover, is not quite true to nature, especially in fig. 1, where the difference in shade between the two generations of augite is accentuated. The mosaic of granules in fig. 2 is only seen, of course, under polarised light.
Fig. 1: St. Helier's Bay.—The ground-mass consists largely of brown glass partly devitrified, and containing numerous laths of feldspar and magnetite. The phenocrysts are chiefly augite, in two generations. The smaller crystals present irregular rounded outlines, and are yellowish-brown in colour; the larger crystals present more regular six-sided outlines, are dark-green, and contain inclusions, especially of magnetite. The phenocrysts of feldspar frequently consist of an outer shell, enclosing brown glass. Others show perfect zonal structure. Striping is absent. There are a few amygdules.
Fig. 2: Deep Creek.—This is a highly amygdaloidal rock, the secondary mineral, which in this case is chalcedony, not only filling the vesicles but also replacing the phenocrysts, none of which appear under the microscope. With polarised light the pale amygdules break up into a mosaic of granules, greys and yellows of the first order being the colours. Each amygdule is bordered by a row of minute granules very regularly arranged, the centre consisting of granules of a larger size. Sometimes streams of small granules connect separate amygdules. Feldspar laths are not numerous in the opaque
[Footnote] * Sir A. Geikie: “Ancient Volcanoes of Great Britain,” vol. i., pp. 61, 62.
ground-mass. A large fibrous mass of yellowish-brown pleochroic bastite appears in the centre of the section. It gives straight extinction along the planes of schillerisation. Under a high power it is seen to be a very pale-yellow mass, with deep-brown prisms arranged in parallel lines, and giving the fibrous appearance. Sometimes little prisms are arranged crosswise, giving a scalariform appearance.
Fig. 3: Cheltenham.—This is a hypersthene-andesite. The ground-mass is hyalopilitic, but the feldspar laths are not very numerous. Grains of magnetite are thickly distributed. The phenocrysts consist chiefly of large striped feldspars, whose extinction angle shows them to be labradorite. Augite also occurs, associated with a rhombic pyroxene, which is probably hypersthene. The colour is paler, however, than in the drawing.
Fig. 4: Onehunga.—This is a very amygdaloidal rock. In the hand specimen it is black, spotted with white amygdules of chabasite, which show under the microscope beautiful fibrous forms. The ground-mass is augitic, with feldspar laths and a little magnetite. The phenocrysts are numerous, especially large striped crystals of labradorite or andesine. Augite is the usual pyroxene, forming very large green six-sided prisms; but pleochroic brownish crystals of hypersthene are also present.
Fig. 5: Wairau Creek.—The ground-mass is typically hyalopilitic, with numerous elongated laths of feldspar, showing flow-structure. Augite is also present in the ground-mass in abundant pale-yellowish grains. Magnetite grains are numerous. The rest of the ground-mass consists of a deep-brown glass, perfectly isotropic.
The phenocrysts consist of plagioclase, but, though binary twinning is common, multiple twinning is very rare, and some of the feldspars are untwinned. Zonal structure is very general. Augite phenocrysts are not very numerous, occurring as pale-green six-sided prisms. The augite and the feldspar contain numerous inclusions of glass and magnetite grains. Magnetite also occurs as large three-sided crystals.
The rock closely resembles an “andesitic basalt,” from the Waitakerei Range, which is in the collection of the University College laboratory, the only difference being that multiple twinning is more common in that rock. It also somewhat resembles an “andesitic basalt” from Eskdalemuir, described by Sir A. Geikie.*
Sir James Hector has always distinguished between the Parnell grit and the Cheltenham breccia, but he placed the latter above the former on the strength of a supposed uncon-
[Footnote] * Proc. Roy. Phy. Soc. Edin., vol. v., 1880.
formability. He wrote: “Great potholes, similar to that now occupied by the North Shore Lake [Lake Takapuna], were formed, and these were filled by Pliocene beds composed chiefly of volcanic agglomerates”*—i.e., by the different outcrops of the Cheltenham breccia, each outcrop marking, I suppose, the site of a former lake. Sir James Hector also gave a section at the Wairau Creek to show that the breccia lay unconformably on the Waitemata sandstones. I have repeatedly examined this spot and can find no section closely resembling the one given, so that I think some other locality was probably intended. The breccia does, indeed, occur at the Wairau Creek, but it appears unconformable to the sandstones from which the fine specimen of Pentacrinus now in the Auckland Museum was obtained. This appearance of unconformability is, I believe, deceptive, partly due to the effects of current-bedding at the base of the breccia, partly to a series of small faults which obscure the section, and partly to distortion of beds of unequal hardness, which is the reason given by Mr. Park. Mr. Lamplugh has shown how a “crush breccia” may be formed where the strata shade off into one another. Here, however, we have beds of quite distinct hardness, and in that case an appearance of unconformability is generally the result of crushing.
In reply to Sir James Hector Mr. Park wrote: “It should, however, be pointed out that wherever the strata occupy a horizontal or undulating position the breccia is seen to be interbedded with and quite conformable to the adjacent beds, and at its base is frequently more or less false bedded with the underlying clays and sandstones. On the other hand, at points of severe local disturbance where the breccia is present the softer and more yielding clays and soft sandstones have in many instances been crushed and contorted and often turned over the more compact, heavy, and unyielding ash-bed, thus giving rise to apparent unconformity.”
Mr. Park's section shows how an inverted-trough fault may produce an appearance of unconformability in beds of markedly unequal hardness. On the weathered face of the cliff the faults and fault breccia are by no means as prominent as they are in his drawing. C is the bed from which Pentacrinus was obtained. In a spot where the underlying sandstones were exposed an inverted-trough fault might still more easily cause a deceptive outline.
It is also quite true, as Mr. Park observes, that the unconformable appearance generally coincides with an area of distortion; but this is not universally the case, as at the Manukau Harbour.
[Footnote] * Geological Reports, 1885, p. xxxviii., woodcut.
I believe that the real reason for this deceptive appearance is to be found in current-bedding. The ancient sea which washed the flanks of the Waitakerei hills was probably studded with islands, of which few now remain. The proximity to the surface, however, of the Palæozoic quartzites and silky slates, some of which the Cheltenham breccia did not cover, is evidence of their former existence. This may have produced violent and conflicting currents which deposited the breccia on the earlier sands and mud-flats in a somewhat irregular way. The Waitakerei hills were not very far distant, and the isthmus of Auckland was then a narrow strait.
The argument for a Pliocene age rests entirely on the supposed unconformability. The argument for a Miocene age rests on overwhelming fossil evidence and also on stratigraphy. The unconformability might not, in any case, be serious, and the appearance may be otherwise explained; but Oligocene fossils, unless derivative, could not be found in a Pliocene bed.
The stratigraphy points to an Oligocene age for the Cheltenham breccia. It may be shown that the breccia is strati-graphically below the Parnell grit; that there is no volcanic bed between the Parnell grit and the Orakei greensand; and that the Cheltenham breccia is consequently inferior to the Orakei greensand—a Lower Miocene bed. It may further be shown that the Orakei greensand is the equivalent in position and fossil contents of the upper parts of the Turanga greensands which overlie the Papakura limestone; and it follows that the Cheltenham breccia is either the equivalent of the lower greensands at Turanga (and therefore at the bottom of the Lower Miocene) or of the limestone at Papakura (and therefore at the top of the Oligocene). The evidence for each of these propositions is given below.
(a.) The breccia is below the Parnell grit. This has been incidentally shown in discussing the question of their identity; but it will now be necessary to give the evidence which leads me to consider the coarse beds at Wairau Creek, Cheltenham Beach, and the White Bluff to be all outcrops of one bed In lithological contents the beds are very similar, differing almost wholly in the varying size of the lava fragments of which they are mainly composed. The arrangement, too, is similar, the coarsest fragments being about a third of the way from the bottom. The agreement in fossil contents is shown in three parallel columns on pages 468–9.
At the White Bluff I had not more than ten minutes to collect, but was fortunate enough to come upon a very fossiliferous patch of Bryozoa. With a more careful search I think it very probable that other Pectens at least would be found; but, at all events, the fossil contents are very similar. The identifications were made chiefly by a comparison with the
plates and descriptions in Zittel and Stolickza's “Orakei Bay Fossils,”* Waters's papers on “Australian Bryozoa,”† and Tenison-Woods's “New Zealand Corals and Bryozoa.”‡ No attempt was made to identify numerous indistinct remains.
(b.) There is no volcanic bed between the Parnell grit and the Orakei greensand. The evidence for this will be given when the Parnell grit is described.§ It may also be noted that if the Cheltenham breccia were between the two it must have been erupted at Orakei since the greensand was, or else it must appear in section between the two, but nearer the Parnell grit, at the Orakei Stream.‖ It does neither. The breccia therefore underlies the Orakei greensand.
(c.) The Orakei greensand is the stratigraphical equivalent of the upper greensands of Turanga. The evidence for this is given better later, and it will be sufficient here to say that the Parnell grit slightly overlies both. Mr. Park found numerous fossils in both, which he compares.¶
I have slightly rearranged Mr. Park's list. A difficulty, however, confronts us in supposing the Cheltenham breccia and Papakura limestone equivalent beds. How is it that the Parnell grit, a thinner bed, extends to the Turanga greensands and the limestone, while the Cheltenham breccia, a thicker bed, does not? If it were certain that the former came from Coromandel and the latter from Waitakerei a sufficient reason is given in that their origins lay in opposite directions. But this is not certain, and I am inclined to offer a different explanation. It will be noticed that round the Waitemata, in early Oligocene times, sandstones and volcanic beds were laid down, while round Papakura limestones and greensands were deposited; so that the two areas have different types of sedimentation, and must have been laid down under different conditions. When, however, we come to Lower Miocene times the same type prevails over both areas, and when a volcanic bed is deposited, as in the case of the Parnell grit, it is deposited over limestones and sandstones alike. In other words, the conditions of deposit had become the same in both areas. This points apparently to a separation, in the earlier period, of the two areas, probably by a land mass; and there is some independent evidence for this. At Mount Wellington, a basalt puy, the Rev. Percy Smallfield found fragments of Maitai slate which had been erupted by the puy; so that the slates are probably at no great depth in this locality.
[Footnote] * “Voyage of ‘Novara,’” vol. ii.
[Footnote] † Q.J.G.S., 1885, &c.
[Footnote] ‡ Part iv. of the “Palæontology of New Zealand.”
[Footnote] § See page 485.
[Footnote] ‖ See page 485.
[Footnote] ¶ Geological Reports.
At Tamaki West Head, in the tuff (or volcanic neck?) of another basalt puy, large angular blocks of Maitai slates, quartzites, and phyllites occur. Lumps of Parnell grit have also been ejected. Some of the Maitai blocks are several feet in diameter, and the rocks must be quite close in situ. Farther east Motutapu is an island consisting mainly of Maitai slates. Much to the west, near the south head of the Manukau Harbour, Maitai slates have also been found. These points may be connected by an almost straight line, and, bearing in mind the lithological evidence and the fact that the Cheltenham breccia seems never to have passed this line, it seems reasonable to infer that in Oligocene times there existed a Palæozoic ridge or chain of islands, whose sunken summits are still traceable, which acted as a barrier to the western deposits. In Lower Miocene times it had sunk beneath the sea, and the Parnell grit spread over the whole area.
The fossil evidence for an Oligocene age is strong. The forms appear to me to represent a position somewhat intermediate between the Orakei greensand and the Papakura limestone; but, as the conditions were somewhat different, the Cheltenham breccia and Papakura limestone may really be contemporaneous. Below I give a list of the fossils so far obtained from the beds. Those marked with an “x” have not hitherto been named as occurring in them. I have not given in full the list of Foraminifera from Orakei, because the Cheltenham forms were not distinct enough to warrant identification. The Barnea from the Cheltenham bed I could not identify, nor was the Orakei Barnea identified by Mr. Park.
|Name of Fossil||Papakura.||Cheltenham.||Orakei.|
|Cidaris sp. (corals)||X||X|
|Crisinta sp. (Foraminifera)||X||X|
|Barnea sp. (Cerithium)||X||X|
I have endeavoured to group them as far as possible so as to show the blending of the faunas of the first and third beds in the second. It will be seen that the Papakura limestone and the breccia have five forms in common, but the former bed probably contains more Bryozoa than Retepora beaniana, which may add considerably to the list. Twenty species of
Bryozoa are common to the greensand and the breccia. Pecten fischeri and Retepora-beaniana are common to all three beds.
The stratigraphical and palæontological evidence are thus both in favour of an Oligocene age. There is evidence also in favour of the supposition that the breccia had its source in the Waitakerei vents; and, since those vents are considered Oligocene, such an origin for the breccia strengthens the above arguments.
My reasons for believing that the source of the Cheltenham breccia was the Waitakerei line of vents are briefly as follows: Firstly, the bed is Oligocene and the vents are Oligocene; undoubted Waitakerei breccias contain a fauna similar to that in the Cheltenham breccia; and the lava in the range is very similar to the lava in the breccia. Secondly, the Waitakerei vents are the only Oligocene vents not far from the breccia, which can be shown to have had a not distant source; and the bed grows coarser in this direction, but does not extend to the east. Thirdly, this supposition explains some anomalies in the distribution of the bed.
I have already given my reasons for considering the breccia Oligocene. With regard to the range I must rely on the observations of others, since I was not able, in the absence of roads and especially in the winter, to explore its forest-clad slopes for myself. The most important observations on the age of the Waitakerei vents are those of Mr. James Park.*
Mr. Park wrote that his work in 1886 tended to show that the Manukau (= Waitakerei) breccias “originated during submarine volcanic outbursts of an intense character, some time during the deposition of the Orakei Bay beds, most probably at the horizon of the Parnell grit and Takapuna [= Cheltenham] ash-bed. At Komiti Peninsula, and further north, on the Wairoa, marine beds, containing characteristic fossils of the Orakei Bay horizon, are interbedded with heavy deposits of volcanic breccias, tufas, and agglomerates, and occasionally sheets of solid lava, consisting of dolerites rich in ohvine, hornblende, and augite-andesites. These can be traced southward to the Hoteo and Kaukapakapa, and an examination of the bush country south of the latter will probably show that they are connected with the breccias of the Waitakerei Range.”
The classing of the Orakei greensand, the Parnell grit, and the Cheltenham breccia as “Orakei Bay beds” is somewhat confusing, and I am ignorant as to what “the characteristic fossils” were to which Mr. Park refers; but, since the out-
[Footnote] * Trans. N.Z. Inst., 1889.
bursts were “most probably at the horizon of (the Parnell grit and) the Takapuna ash-bed,” I infer that they were those species which are common to the Orakei greensand and the Cheltenham breccia—very probably Pectens and Bryozoa. I have not had an opportunity of examining the Waitakerei breccias, but I am informed that immense numbers of Bryozoa are found in them; and this is the most striking and characteristic feature of the Cheltenham breccia.
A description has already been given of the Cheltenham lava. Besides examining the slides of the Waitakerei lava in the Auckland University College laboratory, I made several myself from lava in situ, and found the lava very similar to that contained in the breccia. Naturally, all the outcrops of the bed are not likely to owe their origin to the same vents, since several vents along the chain were, no doubt, active simultaneously. Where, however, we can ascertain approximately the position of the vent from which the materials of the bed at any one outcrop were derived the lava in situ shows a striking resemblance to the lava in the bed, which is all we can expect. Till the Waitakerei eruptions have been studied, more than a general similarity need not be looked for. I may, however, give one case of more certainty, and therefore more interest.
The outcrop of the breccia at the White Bluff is not many miles from the range, and is, moreover, at the southern end of the chain, so that the southern vents may be considered as its source. In the bed there are several varieties of lava, but the commonest are—(1) A purple earthy rock with abundant phenocrysts of kaolin; (2) a black amygdaloidal rock with bright unaltered feldspar phenocrysts; (3) a dark-brown or chocolate rock of a somewhat holocrystalline appearance since the phenocrysts of feldspar are very numerous, and large black phenocrysts of pyroxene also occur. These three kinds of lava are found to be plentiful in situ in various localities between Big and Little Muddy Creeks, about six miles to the westward.
The Waitakerei Range is, moreover, the only chain of Oligocene vents near at hand, and the breccia cannot have come from a distance. The size of the included fragments, varying from 1 ft. to 4 ft. in diameter, is, it seems to me, conclusive. I have also found numbers of elliptical or round volcanic bombs, which are not likely to have come a great distance. The bed cannot often be seen in successive outcrops, each nearer than the last to the Waitakerei Range, but where this can be observed the nearer outcrops are always the coarser. St. Helier's Bay, for instance, is about twice as far from the vents as the White Bluff, and the coarseness at the latter is much more marked than at the former spot.
Lastly, such a source as I suppose explains anomalies of distribution which I shall now describe. Mr. Park wrote that the ash-bed had a linear extension from Parnell to Whangaparaoa Peninsula, thirty miles from Auckland, and I shall assume that the volcanic breccias of that peninsula are really extensions of the Cheltenham bed. In that case the coarsest outcrop of the breccia is at the White Bluff, but at Wairau the breccia is coarser than at Cheltenham, which lies midway between White Bluff and Wairau. Again, it is coarser at Deep Creek than at Wairau or Okura, yet Deep Creek lies midway between Wairau and Okura. Or let us consider the variations in thickness. At White Bluff it is 30 ft., at Cheltenham 25 ft., at Wairau 30 ft., at Deep Creek 40 ft., at Okura 4 ft., at Whangaparaoa 20 ft., each of these places being farther north than the preceding one.
It seems to me that these wide variations in both thickness and coarseness must be due to a variety of sources, in a chain of vents. Until we have far fuller fossil evidence than we at present possess it seems impossible, or at least inadvisable, to attempt to separate these beds, which we may judge from their coarseness to have all come from the Waitakerei vents, the fossils in the beds, from White Bluff to Onehunga, confirming this. To attempt the separation it would be necessary to know the position of the vents, of which we are quite ignorant. In the meanwhile we may group the beds together as the “Cheltenham breccia,” regarding this really as a number of breccias formed at much the same time under much the same conditions, but derived each from its own vent or group of vents; for we know that the Waitakerei Range extends northward at least as far as Whangaparaoa, and that there are other andesitic volcanoes beyond that which may be of the same age. If we accept this origin for the breccia our difficulties vanish. The volcanoes of the Waitakerei chain were not equally active, equally powerful, or exactly synchronous. Some outcrops of the resulting breccias would be thick and coarse, others thinner and finer, while here and there breccias of different origin would be mingled together. Nor would the vents be quite in a line; some might be to the east of it, some to the west. Some would throw out mainly scoria; others, with greater explosive energy, would eject large blocks. The thickness would probably vary more than the coarseness, since no outcrops are far from the main line, and this we find is actually the case. I believe the Okura breccia may pretty safely be separated from the Cheltenham, but beyond that it is impossible to go; and perhaps it is better, on the whole, to use the term “Cheltenham breccia” for the present in the wide sense I have given to it.
The outcrops of the breccia at White Bluff and Wairau have already been described in sufficient detail. The other outcrops now to be described are as follows: (1) St. Helier's Bay; (2) Cheltenham and Narrow Neck; (3) Deep Creek; (4) Okura; (5) Whangaparaoa.
The outcrop at St. Helier's Bay is a reef, covered at hightide, about 20 yards from low-water mark. It is a coarse bed, with large rounded and angular fragments, and contains Pecten zittelli, Pecten burnetti(?), and Bryozoa too much weathered for identification.
Cheltenham Beach is the first outcrop north of the harbour, and is the typical one (Plate XXXVIII., fig. 2.) The bed has an easterly dip, and comes down along the face of the cliff, which extends at right angles to the above section. The breccia is thus exposed for several hundred yards. The cliff then again turns east and west, and at the head of the bay the dip has increased from 30° to 70°. Round the headland there is violent dislocation and a fault. Beyond this the sandstones show a beautiful example of a dome, the centre of which is at the middle line of the beach. A few hundred yards beyond, the north-westerly dip brings down a volcanic bed. This is much thinner than the bed at Cheltenham—only about a quarter of a mile distant in a direct line. It is, however, equally coarse, and I see no reason to suppose it to be a separate bed. The only fossils obtained were some weathered Bryozoa. This outcrop is very remarkable, owing to the fact that in the breccia as seen in the face of the low cliff there is evidence of very complete decomposition, while where it is exposed to the waves it is quite fresh and hard. At the former place it has weathered to a creamy white and is clayey to the touch; the lava is pale-grey, scarcely distinguishable from the matrix, and quite soft and rotten, so that the bed might be mistaken for a mudstone. As one follows it towards the water's edge it hardens and darkens, till at low-water mark it exactly resembles the outcrop at Cheltenham, the lava here being dark-grey or black, and giving a ringing sound when struck with the hammer, though at the cliff one may pull it to pieces with one's fingers.
Crossing the bay, along the shore of which there is nothing more than a bank a few feet high, the dip continues regular, till at the north headland the breccia is again met with in a long reef separated from the cliffs by a fault. It runs out into the sea for several hundred yards. I found no fossils except Bryozoa. It seems quite possible that the reef may be an extension of the fault. The outcrop at Wairau Creek has been mentioned in discussing the age of the bed. It is the next outcrop.
About seven miles from Wairau Creek there is another outcrop of the breccia. The strata near Deep Creek have a southerly dip, which brings up the volcanic bed at the south head of the bay. At the north head it forms the mass of the cliff, and also a small island 50 yards from the shore, which is completely composed of it. The actual thickness is nowhere seen, as a fault occurs beyond the bay, but the bed must be more than 30 ft. thick. It is rather coarser than at Cheltenham, and contains numerous Bryozoa. The weathering of the upper parts is very noticeable. The upper 8 ft. weathers to a rich red earth with concentric markings (Plate XXXVIII., fig. 3).
On the north bank of the Okura River there is an outcrop of a coarse volcanic breccia interbedded with shale and sandstone. In texture it resembles the Deep Creek bed, but it is not more than 4 ft. in thickness. The dip takes it up above the cliff-line. The Okura River forms a kind of V with the Wade River, the apex being directed seawards. On crossing the Wade River one is at the foot of Whangaparaoa Peninsula.
Whangaparaoa Peninsula, thirty miles from Auckland, is about thirty miles round. This journey had to be done in a day along a rugged coast, where it was sometimes difficult to get along the rocks, so that I had little time for observations. Unfortunately, too, being pressed for time, I omitted the end of the peninsula from the journey, and this apparently was the locality from which Hochstetter's section was taken. The cliffs on the south are composed of hard sandstones with layers of shale, and at intervals a volcanic breccia resembling in all respects the Okura breccia is interbedded with these. It is perhaps slightly coarser than at Okura, but the thickness is very regular—about 4 ft. The first point at which I observed it was in the cliff opposite Kohanui Island. Large blocks have fallen from the breccia, which lies on a shale bed about 30 ft. from the foot of the cliff. No doubt the shale has become slippery, allowing large blocks of the hard breccia to slide away and form an admirable protection at the foot of the cliff. The fragments of lava contained in it are a compact grey rock with feldspar phenocrysts. It is apparently an augite-andesite, and resembles fragments from Cheltenham breccia. At Korimai Bay there is a very similar outcrop of the same bed, and there are occasional outcrops of the bed on the northern shore, but part of this coast I examined in twilight. Along the beach I noticed occasional fragments, well water-worn, of augite-andesites.
Mr. E. Wilson informs me that at Mahurangi Heads, about ten miles to the north of the peninsula, there is an outcrop of a very similar bed; but it is coarser, lumps of lava
“from the size of an orange downwards” occurring in a “clayey matrix.” Some of the fragments were forwarded to me. Most of these are augite-andesites, but one specimen appears to be a dolerite. The augite-andesites closely resemble the andesites in the peninsula breccia. I am informed that this coarse breccia is conformably overlain by sandstones, and may be traced seven or eight miles farther north, cropping out at Omaha and Matakana.
In addition to these coarse beds, I observed in the cliffs of the peninsula ash-beds of a much finer grain, not unlike the Wairau tuffs.
In conclusion, I add Hochstetter's remarks on this locality: “The peninsula of Whangaparaoa, which I visited from Auckland, consists principally of the same Tertiary strata which constitute the isthmus of Auckland. The steep cliffs in the hills show the horizontal strata clearly exposed: at the bottom generally fine-grained sandstone in layers 6 ft. to 8 ft. thick, and over these a thin stratum of clay marl. Very frequently there are intermediate strata of volcanic tuff, which is partly developed as a fine-grained sandstone, and partly coarse-grained as a breccia, consisting of fragments of trachyte, dolerite, and basalt. At the places where coarsegrained breccias and conglomerates appear very striking local disturbances of the strata are noticeable. A very instructive section is afforded by the north (north-east?) shore of the peninsula.* a is a tuff mass showing in places a very coarsegrained conglomerate of fragments of volcanic rocks, and containing much augite in little shining crystals, and, besides augite, little twin crystals of glassy triclinic felspars. This mass appears as an eruptive formation which has penetrated between the sandstones c and clay-marl d, torn them asunder, broken them by lateral pressure to the westward, and forced them out of their original horizontal position At b the tuff is fine-grained, and in places full of Foraminifera and Bryozoa. A smooth Terebratella (Waldheimia lenticularis) was also found here enclosed in the tuff, so that there can be no doubt we have in these volcanic tuffs the products of submarine eruptions, with which the volcanic outbursts commenced in the Tertiary period.”
I have quoted this passage at length, because it gives so admirable an illustration of the manner in which these Tertiary breccias may be distorted. An equally good example will be given when the Parnell grit is described. Hochstetter does not attempt to explain how the volcanic breccia was injected, and I am at a loss to explain it.
From the description of this bed, and its contained fossils,
[Footnote] * “Reise der ‘Novara,’” quoted in Geol. Surv. Repts., 1885, p. xxxvi.
and my own examination of the peninsula breccias, I cannot but think that, though these are doubtless outcrops of a volcanic bed which owed its origin to Waitakerei Oligocene vents, those vents were not identical with that which produced the Cheltenham breccia. Probably it was equally near (for the lava fragments are quite as coarse), but from the thinness of the bed one is led to infer that the vents which formed it were smaller, or that they continued active for a comparatively short period.
The Parnell Grit.
The Parnell grit, like the Cheltenham breccia, presents a somewhat bedded appearance, due to the linear arrangement of fragments of similar size. In this case, however, the bands are much more regular, and the coarsest fragments—the size, perhaps, of marbles—are all at the bottom, from which point the bed grows gradually finer till it shades off into a sandstone. The outcrops of the Parnell grit everywhere present the most beautiful examples of the shading-off of one rock into another. The red rounded lapilli of the lowest layers are gradually replaced by smaller ones, and these by yet smaller, till at length they can no longer be observed with the naked eye, and on a fresh fracture the rock has all the appearance of a blue sandstone; nor is it possible to say where the sandstone begins and the breccia ends. The lapilli are very uniform in size in each layer, which seems to indicate a distant origin. The average thickness of the bed is about 18 ft. Fossils do not seem to be as abundant as in the older beds.
The weathering is extremely characteristic and useful. In the coarser parts it is similar to that of the Cheltenham breccia, but in the upper parts concretionary or spheroidal structure is usually developed. Round the shells of brown iron-oxide the dark colour which the bed usually presents is absent, so that the concentric layers are plainly visible. This type of weathering seems to be almost confined to this bed, none of the Waitemata sandstones exhibiting it; and it is invaluable as a means of detecting the presence of the bed in inland outcrops, where the colour has generally all been leached out and the bed is left an (apparently) white crumbly sandstone. Under sea-water neither this bed nor the Cheltenham breccia weathers more than a few inches, and the feldspars remain comparatively fresh.
As at Cheltenham, a very common feature of the bed is the occurrence of zeolite “dykes,” dividing the surface of the grit into irregular polygonal plates. These dykes, when split, frequently show beautiful dendritic manganese markings. The markings closely resemble those formed by moist emery-
powder when, after grinding a section, one slowly withdraws the slide at right angles to the iron plate, the markings occurring both on the plate and on the slide. Possibly the dendritic markings on the zeolites were formed in an analogous manner.
The lava fragments are so small and so oxidized that I was not successful in making any microscopic sections. My sections of the grit were scarcely more successful, the rock crumbling away before it was thin enough to be of much service. In one or two instances fragments of lava could be recognised in the sections, containing feldspar and six-sided augite phenocrysts and probably an augite-andesite.
Among the crystals in the tuff corroded quartz grains occur, which may be due to the mingling of sandy sediment, or, since Sir James Hector says that the grit contains fragments of trachytes, may be derived from them or from more acid rocks. By far the most numerous crystals are small oblong feldspars sometimes altered and milky, at other times fresh and bright, with good cleavage. These are very plentiful throughout the bed. A few broken fragments of augite crystals are also present, but the large perfect augites described as occurring in the Cheltenham breccia are conspicuous by their absence. The grit is largely composed of fragments of greenish sandstones and slates which may be Palæozoic rocks. Large blocks of sandstone are absent, and current bedding is unusual. Iron-pyrites is plentifully disseminated in bright-yellow flecks. The black matrix is often studded with scarlet scoria and white feldspars, and forms a handsome rock. From the plentiful occurrence of feldspar and augite the grit may be best described as an augite-andesite tuff.
I do not consider the source of the Parnell grit by any means as certain as that of the Cheltenham breccia, but, on the whole, the evidence is perhaps in favour of a source near Cape Colville. Before giving what evidence there is in support of this I must freely admit that the grit may have come from the Waitakerei vents, like the other volcanic beds of the series. But in the first place it is a good deal younger than the Cheltenham bed, as is shown by the fact that it crosses the Maitai ridge, which apparently was above water when the former bed was deposited; and so it is quite possible that the Waitakerei vents may have become quiescent. In the second place, it is a bed with a very wide distribution, extending from Ponsonby to Turanga Creek, from Manukau to St. Helier's. Yet it is not a coarse breccia. Such a widespread bed must have been, one would think, the result of very violent eruptions, and if the eruptions at Waitakerei were very violent the bed at the Manukau ought to contain some
coarse lumps. But, if it came from Coromandel, a great outburst might result in just such a bed at so considerable a distance. In the third place, it grows coarser, on the whole, in an easterly direction. At the Manukau the bed at the White Bluff, though evidently the same, is yet rather finer than at St. John's, to the east. I cannot say that I have observed any increase of coarseness between Ponsonby and Parnell, but between Parnell and St. John's College there is a distinct increase. At Tamaki Head the bed is coarser than at St. John's, while the outcrop in the reef is, on the whole, intermediate between the two. It is very noteworthy that Mr. Park was so struck by the increase of coarseness at Howick that he described the bed there as “much coarser” than at Parnell. Had he seen the outcrops at St. John's, St. Helier's, and Tamaki Head he would hardly have noticed the increase, so gradual is it; but it becomes noticeable when we compare places far apart. Ponsonby is eight or nine miles from Howick. At a spot between Little Muddy Creek and Avondale I found what appeared to me an outcrop of the Parnell grit. The bed weathered in concretions, and was, as usual near vegetation, leached of its colour, and crumbly. It was much finer than near Auckland, and only 10 ft. thick, and the sandstones with which it was interbedded seemed to be lying on Waitakerei lavas, but this I could not decide. It is so like the usual outcrops of the Parnell grit that I see no strong reason for doubting its identity. If this be accepted, the Parnell grit evidently did not come from the Waitakerei vents; and I think the evidence from coarseness is entirely in favour of its having come from the east, not the west. Lastly, we have the opinion of so excellent a geologist as Sir James Hector, already quoted: “The Parnell grit, as far as I have seen, contains no fragments of the volcanic rocks of the district, but is greensand, with well-rolled pebbles of cherty slate, quartzite, and other Palæozoic rocks, and occasional fragments of old trachyte and basic rocks of Cape Colville.” I might add that the volcanic inclusions are invariably in the form of lapilli, full of steam-vesicles, such as one would expect to have come from a considerable distance.
The age of the Coromandel volcanic gold-bearing series is a matter of dispute; but Mr. Park, who has studied them, is evidently convinced that they are about the same age as the Parnell grit. He had in 1889 placed the grit in the Upper Miocene, but for some reason he changed his mind, and wrote in 1897: “Judging from the fossiliferous Parnell bed in the Upper Eocene Waitemata marine series, on the shores of the Waitemata, with its contained fragments of andesite and coarse ash, the author is of opinion that the eruptions which originated these gold-bearing rocks began in the Upper Eocene
and continued down to the Lower Miocene period.” In my opinion, Mr. Park is here confusing two beds; but, be that as it may, he seems to consider that the Coromandel Peninsula eruptions began at the horizon of the Cheltenham breccia, but lasted for a long while, and this is all that is required in the supposition that the Parnell grit came from Coromandel.
The most interesting question relating to the Parnell grit is its stratigraphical position. Most observers have supposed that the grit overlies the Orakei greensand, but apparently on no very good evidence. Captain Hutton contributed a paper in 1884 to the New Zealand Institute in which he concluded that the relative position of the beds was uncertain, and he even wrote: “To the east of Parnell, between Resolution Point and Hobson's Point, there is a break across Hobson's Bay in which nothing definite can be seen. It is therefore quite impossible to say from stratigraphical evidence whether the beds at Hobson's Point are above or below the Parnell grit.* It is not really at all impossible, and it seems to me that Captain Hutton implied as much when he assumed that the Hobson's Point fossiliferous greensand is an extension of the Orakei bed. No one doubts this; but it is evident that if the greensand may be traced from Orakei to Hobson's Point it may be traced farther, and thus new evidence may be supplied. The Parnell grit may also be found cropping out farther east. As a matter of fact, both the greensand and the grit do occur at many localities eastward.
Before giving this new evidence it will be well to review Mr. Park's opinion, which was the reverse of that here adopted. Considerable support is given to Mr. Park's opinion by the sections which he published, but I am unable to agree with Mr. Park's interpretation of the stratigraphy. The dips are, in my opinion, sometimes the reverse of those given by Mr. Park, who wrote in 1889†: “As bearing on the relation of the Parnell grit to the Orakei Bay bed, I may mention that during my last visit to St. George's Bay I found a number of Orakei fossils in the flat irregular calcareous gritty cornstones at the foot of the cliff on the west side of the bay. These cornstones are only exposed at low water, and occupy a position some 15 ft. or 20 ft. above the Parnell grit. The fossils collected at D were Pecten fischeri, Vaginella, Orbitolites, and a number of small corals. The occurrence of Orakei Bay fossils in this position would tend to show that the Parnell grit is inferior to the Orakei Bay beds; but, if the
[Footnote] * Trans. N.Z. Inst., 1884.
[Footnote] † Trans. N.Z. Inst., p. 399.
evidence is not sufficient to prove this, it shows that these two horizons are at least not far separated from the Orakei greensand.”
Now, in the first place, Pecten fischeri might occur at any horizon from the Oligocene limestone at Papakura to the Miocene greensand at Orakei, if it has not a greater vertical range. But, be that as it may, I cannot agree with Mr. Park that the Parnell grit underlies the cornstones, as shown in his section.
Mr. Park was of opinion that the bed could be seen dipping under the Mechanics Bay beds; but, although the grit may certainly be seen in the floor of the bay, I think that a fault separates it from the beds at Mechanics Point. This fault dips, I believe, as Mr. Park's section shows it (only the plan can actually be seen), but I would make it a normal fault, Mr. Park a reversed one; and, in my opinion, the grit is superior to the Mechanics Bay beds.
But the most important point in which I differ from Mr. Park is the amount and direction of the dip at B in his section. A photograph was taken for me, which shows that the dip is a low westerly not a high easterly one.
Mr. Park shows another section of less importance at Hobson's Bay, where, again, I must differ from him, not as regards the amount of dip so much as the direction, which here, again, I consider to be the reverse of that given in his section.
At Hobson's Bay, however, there is no fossil evidence, and the dip which Mr. Park gives to the grit is suggestive of a position inferior to the Orakei greensand at C of his section, but only suggestive; and Mr. Park did not seek to establish that position from this section.
My observations do not prove that the Parnell grit lies above the Orakei greensand, for there is a mile of mud-flat (covered at hightide) between Morrin's Point and Resolution Point. If this were all the evidence a superior position for the Parnell grit would be only suggested; the fuller evidence lies to the east, where both the grit and the greensand may be followed for several miles.
Mr. Park found a bed “resembling” the Parnell grit at Howick, but apparently doubted the identity. He says it is coarser; so that the grit seemed to him to get coarser in two opposite directions—Cheltenham and Howick. Besides, he found “lumps of limestone” at the base of the bed, and thought it Kaipara limestone. Kaipara is to the north-west, very much nearer the grit at Parnell and Cheltenham than at Howick, so that it was difficult to see why the limestone lumps were not at the former places, although found at Howick.
Except for this notice the Parnell grit has not hitherto
been described to the east of Point Resolution. I have been successful in finding seven new outcrops in this direction, and they supply a good deal of new stratigraphical information. Some hitherto unrecorded outcrops to the west are also full of interest.
Round Point Resolution the cliffs grow low, and are covered with scrub. Here and there, however, an outcrop of the strata may be seen along the cliffs to Newmarket. The beds all have a westerly dip. Not far from Newmarket the Parnell grit can be seen, so that it is apparently well above Morrin's Point beds here, and rising in that direction.
Leaving this doubtful locality, we come to outcrops where the evidence for a superior position is more important. There is, near St John's College, a very fine exposure of the Parnell grit. The College itself stands on a high ridge 300 ft. above the sea. Numerous small streams have cut their way down in a north-westerly direction to the old crater-lake of Orakei. The chief stream rises at the College, and, cutting down through shales and yellow sandstones, which are dipping as the bed of the stream, but at a lesser angle, reaches the Parnell grit. Here there is a considerable waterfall: the underlying shales have been eaten away much more rapidly than the hard volcanic bed. It is at this waterfall that the grit is so well exposed. It is from 15 ft. to 20 ft. thick, and rather coarser, in the lower parts especially, than at Parnell. There is a large amount of iron-pyrites in it, and very numerous laths of feldspar. I could not find any fossils. The grit is overlain by the yellow-and-white sandstones, and overlies conformably, with no sign of current-bedding, a layer of shale. Beneath this is a calcareous grey sandstone, very hard and full of fantastically shaped concretions. In the concretions of a very similar sandstone at Kohimarama I obtained some well-preserved lamellibranch shells, about the size of Venus, which I have not been able to identify. These sandstones are at about the same horizon.
Plate XXXVIII., fig. 4, shows the position of these beds. At 4 in the section occurs the waterfall; the stream has cut right through the grit here, just before the latter rises north-west. The outcrop between A and B is not easy to find. It may be seen in a well close to the Presbyterian Church. There is one break in the section, at 5, where a modern tuff volcano has covered the beds and distorted them close to the point of eruption. I do not think, however, that this eruption disturbed the general dip of the beds. The Orakei greensand should come in between beds 2 and 4. Everything there is covered with very dense gorse. The Wairau tuffs are seen in the bed of the creek when the tide is out and a freshet has scoured away some of the mud.
I examined the Orakei tuff to see whether it would throw any light on the question. It is chiefly composed of fragments of basalt (Mr. Park mentions trachyte; I could only find hard black scoriaceous lava, with olivine), but here and there are fragments of grey sandstones and shales, and occasionally of the Wairau tuffs. There are no fragments, as far as I could see, of the Parnell grit. If this thick bed had been below the Orakei greensand, fragments would in all probability have been thrown out. A beautiful illustration of such an event occurs in the case of a similar outburst at Tamaki Gulf, where a modern volcano has broken through the Parnell grit and contains large fragments of it scattered everywhere through the tuff. The Parnell grit is an easy rock to recognise, and as it is hard it would not be blown to dust if the Wairau tuffs escaped. But fragments of these tuffs are included. Now, these are below the grit, so that the grit was, no doubt, all denuded away before the outburst—at least, that portion of it above 5. In that case it must have been at a good elevation, and therefore probably above the Orakei greensand. It was at C that Hochstetter collected from the greensand.
The outcrop of the Parnell grit, forming a reef in the harbour near the Bean Rock Lighthouse, throws no light on the stratigraphy.
The next outcrop is at the west head of the Tamaki. Before referring to this it will be advisable to trace the Orakei greensand in the same direction. At the bridge which crosses the outlet of the sunken Orakei crater to the sea the rocks exposed in the low cliffs are the Orakei tuff beds. These are seen a little further on to be quite unconformable to the Waitemata sandstones. Interbedded with these is the Orakei greensand. Here it is a greenish sandy bed which thins out completely in both directions—a lenticular mass; but it appears again at a little distance on both sides. This patch is the most fossiliferous outcrop, yielding more than forty species of fossils in a few yards, though the bed is only a couple of feet thick. Proceeding round the cliffs towards St. Helier's Bay the bed is next seen, beyond a fault, as a reef separated from the shore by 30 yards of deep mud and covered at hightide—here, again, richly fossiliferous. A little beyond the west head of Okahu Bay the strata have a westerly dip, which brings up the greensand in a gentle slope across the face of the cliff. Pecten zittelli, Pecten fischeri, Gastropods, and Bryozoa are the commonest forms contained in it. A fault exists in the middle of Okahu Bay, or the dip changes, for on the east side of the bay the bed is seen dipping easterly, and is again seen at the Bastion, where it has an easterly dip and passes down below the water. From this point to Tamaki West Head I
have not observed any outcrop, so that I cannot say with certainty that the beds are identical. I think, however, we may fairly assume that they are. The lithological characters are the same, and peculiar to these beds; both are somewhat gritty greensands with small red patches of volcanic fragments. The fossils at the Tamaki Head bed are Pecten zittelli, Pecten fischeri, and most of the Orakei Bryozoa named by Hochstetter in the “Voyage of the ‘Novara.’” The bed can be seen at intervals along the cliffs of the Tamaki Gulf, till towards Panmure we reach pumice sands unconformable to the Waitemata series. In these outcrops I have only found Bryozoa, but they are much more numerous than at Orakei even, though the same species.
I have given a somewhat full description of this bed—First, because it is, so to speak, a central line in the Waitemata beds from which other horizons may be worked out; and, secondly, because, though it is undoubtedly best described as a sandstone, it yet contains volcanic fragments, and is therefore connected with the volcanic beds of the Waitemata series.
At Tamaki West Head occurs one of the most interesting sections in the vicinity of Auckland. It is interesting from several points of view. In the first place, it supplies a section in which the Parnell grit and the Orakei greensand both occur, and is the only section I know of in which they do. It is also the spot whence Major Heaphy's section was taken, a section which has since appeared in most text-books on volcanoes. Moreover, it is in this locality that the volcanic neck(?) occurs, with large blocks of Maitai slates and quartzites.
Mr. Park has given a section of this most interesting locality.* Here, again, however, I am not able to agree with him regarding the dip of the beds at the west head. A photograph kindly taken for me by Mr. W. Satenby will exemplify my views. Mr. Park, moreover, omits from this part of his section the most prominent bed, the Parnell grit, and also the Orakei greensand. On the western part of his section he omits the high sandstone cliffs, and therefore, of course, the Parnell grit. He also writes that the beds are “undulating gently”; but, in my opinion, this is one of the most contorted spots on the whole isthmus.
In Plate XXXVIII., fig. 5, A on the section is the Tamak West Head, B a bluff not far from St. Helier's Bay. At C there is distortion and small anticlines, not shown in the section because they are on a small scale though very perfect. 1 is the Parnell grit, 2 the Orakei greensand, 3 sandstones and shales (but the beds are more numerous in the actual section), and 4 a recent tuff crater. This section is an extremely
[Footnote] * Geological Reports, already cited.
interesting one. It is said to be the one drawn by Major Heaphy* to show the strata through which a vent is forced, dipping down towards the vent. If it is the same, denudation must have been very active, since the crater is gone and the levels altered. I know, however, of no similar section in the Auckland cliffs, and the blocks shown in Judd's “Volcanoes” present much the appearance of the great blocks of Maitai and Parnell grit and sandstone which have been emptied at 4, or perhaps are the relics of the agglomerate in the old volcanic neck. 4 is the tuff. Numerous landslips have occurred here, obscuring the relations between the more recent tuff and the Waitemata beds. A plan of this tuff cone is given by Hochstetter.† It forms a beautifully stratified cone, each layer composed of basalt fragments in a clayey matrix; and, as already mentioned, the agglomerate of the old neck(?) consists largely of green Maitai slates and phyllites often siliceous, and Parnell grit. There can be no doubt that the basalt vent burst through the grit. This forms a reef parallel to the shore opposite the cone, and at A the reef has curved round to the shore and dips up towards the cone. The chief interest in the section, however, lies in the fact that here at last we have the grit and the greensand together. But, unfortunately, even here their relation is obscured.
Near B the grit is quite white and crumbly, but weathers in characteristic spheroidal fashion, so that it can easily be recognised from below. Near A I am at a loss to account for the appearance of the beds. Just below the grit is a sandstone, and the grit seems to lie unconformably on this sandstone, which is several feet thick at the first fault, and thins quite out at the second. Stranger still, though the greensand is lost at the fault, and, so far as I have seen, does not appear below the grit towards B, at A the grit passes right over the fault without any dislocation, and, rapidly increasing in dip to over 50 °, passes down to the sea round the head. The only explanation I can give is that the eruption has driven the harder bed over the softer ones for some distance. The objection to this supposition is that, although the appearance in the section seems to fit in with it, just round A the grit seems quite conformable to the underlying sandstone, which in turn is only a few feet about the greensand. But I think such a position for the grit impossible, for then at Orakei we should see it above the greensand, while at Parnell we might expect to see the greensand below the grit. As we do neither there must be a considerable thickness of beds between them—at least 50ft., I should say—and the appearance beyond A
[Footnote] * Judd: “Volcanoes,” p. 165.
[Footnote] † “Voyage of the ‘Novara,’” vol. i:, Geology.
must be deceptive. However we explain matters, it seems certain at any rate, that the grit is above the greensand. It is, perhaps, possible that this bit of the sea-floor was above water for a short time in the Miocene period, and that the grit is really unconformable; but I am inclined to think the explanation lies in the proximity of the basalt vent.
The grit forms the face of the cliff round the point, and this is probably one of the best places near Auckland for seeing its structure. The zeolite veins are well developed in particular. The fault is only inferred from the horizontal position of the sandstone at B, as landslips have covered the face of the cliff.
There is one other spot at which light is thrown on the relative positions of the grit and greensand—Maungamaungaroa Bridge. Mr. Park concluded from the fossils found here. that these greensands were the equivalent of the Orakei greensand. I do not know at what spot exactly Mr. Park found these fossils, but interbedded with yellow sandstones and shales, and above the greensands, occurs an outcrop of the Parnell grit, very much weathered and easy to miss, containing Fasciculipora ramosa.
It may, then, be considered as highly probable that the Parnell grit is above the Orakei greensand, and even probable that it is considerably above. This fixes the age of the grit, since by a consensus of opinion the Orakei greensand is classed as Miocene, the Geological Survey alone considering it earlier.
Professor Rupert Jones, who examined the Foraminifera, thought them to indicate a “late Tertiary age” for the bed.
Herr Karrer, in the palaeontological section of the “Voyage of the ‘Novara,’” made the bed the equivalent of the Vienna basin—i.e., Miocene.
Professor Martin Duncan identified them with the Mount Gambier series in Australia—Miocene.
Professor Hutton, the first of New Zealand palaeontologists, examined the evidence generally, and came to the conclusion “that the evidence, both stratigraphical and palaeontological, is altogether in favour of the Orakei Bay beds belonging to the Pareora system.”
Since these beds are almost universally classed as Miocene, and from the Papakura limestone to the highest Waitemata sandstones the series apparently has no break, the greensand may fairly, I believe, be put at about the middle of that series. In that case the Parnell grit is Upper or Middle Miocene.
In dealing with the source and age of the grit I have already had occasion to describe some of the outcrops.
These I shall not again refer to. The following is a list of the outcrops:(1) Shelly Beach, (2) St. George's Bay (3) Judge's Bay, (4) Newmarket, (5) St. John's College (6) St. Helier's Bay, (7) Tamaki West Head, (8) Howick, (9) Maungamaungaroa Bridge, (10) White Bluff, (11) Cape Horn, (12) Little Muddy Creek, (13) Point England. (14) Blockhouse Bay(?).
At Shelly Beach the grit forms a synclinal. It has been described by Mr. Park, and I have nothing to add except with regard to the fault on the east of the section. I do not feel at all sure that this fault dips easterly, but I could not see the line of fault, owing to landslips and the fact that the bank is not high at Shelly Beach Road.
At Acheron Point, half a mile to the east, occurs the Ponsonby tuff. To that bed the dip is regularly west, so that there is at least 100ft. of strata above the Ponsonby tuff at Shelly Beach. Now, the stratigraphical relations of the Ponsonby tuff are very puzzling, but it will be shown later that it is probably a little above the Cheltenham breccia, and therefore much below the Parnell grit. But here, if the Parnell grit is above it, it must be at least 100 ft. above. I see no objection to this, and am therefore inclined to think the Parnell grit is above the horizontal strata which form the Auckland hills, near the wharves, at Freeman's Bay. Hobson Street, Fort Britomart, and Mechanics Bay. Mr. Park strengthened this supposition by finding a few fossils, also found at Orakei Bay, in the Mechanics Bay bed. Both Professor Hutton and Mr. Park, however, believe that the grit overlies these beds.
Round St. George's Bay the grit dips under the sea. On the west of the following bay (Judge's Bay) the beds are much contorted. A section is given by Hochstetter. On the east of the bay the Parnell grit is beautifully exposed. This is the best locality for seeing the gradual shading-off of the coarser grit into a blue compact sandstone. Blocks have fallen from all parts of the bed, and every variety of texture may be observed. There are numerous zeolite veins running across the bed.
The next point at which the grit occurs (omitting places already described) is a reef not far from the Bean Rock Lighthouse. At the lighthouse itself the rocks are scoriaceous Auckland basalt. This cannot, I think, be derived from the North Shore puys or Rangitoto Island (a basalt volcano), since there is a deep channel between in each case, so that it probably marks the site of an ancient puy, perhaps a submarine one; but more likely its present position is due to the submergence of the old Waitemata River, which has led to the formation of the Waitemata
Harbour. This puy stood, perhaps, on the bank of the river near its mouth.
About a quarter of a mile to the east and nearer the shore a long low reef is exposed, which I visited in a boat, with the object of seeing whether the outcrop of the Cheltenham grit was here. The reef proved, however, to be the Parnell grit, lacking, as usual, coarse fragments, and unfossiliferous except for a few Bryozoa, which my friend Mr. E. K. Mules discovered, but which were too weathered for identification. There is a second smaller reef parallel to the first and only a few yards distant. The second reef is probably only another part of the grit. This outcrop of the grit is about as coarse as that at St John's College.
The next easterly outcrop is at Howick. The Tamaki Gulf is crossed by a bridge at Panmure, about three miles from the mouth. Crossing this bridge, and making for a. point near the mouth, it is easy to miss a long headland of cliffs which forms the east head of the gulf. Viewing this headland from the terminus of the road, it seems a small one, and the strata appear horizontal, so that Mr. Park, observing it doubtless from this point, wrote that the strata “are horizontal to the Tamaki, and consist of sandstones.” I fell into precisely the same error, and it was not until several weeks later that I had an opportunity of seeing the real extent and position of the beds from the harbour. Unfortunately, I was too far distant to make exact observations, and I was not able to revisit the spot. but it was evident that not only are the beds much disturbed, but that at intervals there occurs a bed much resembling the Parnell grit.
Beyond the bay to the east of these cliffs occurs an undoubted outcrop of the grit. Mr. Park, who examined the same bed rather farther along, says that it is coarser than the Parnell beds, and “contains lumps of limestone in its lower parts.” It certainly is coarser than the grit at Parnell, but very little coarser than the grit at the other side of the Tamaki, which Mr. Park did not observe. It is about 20 ft. thick, and traversed by veins of calcite. These in places have thickened into lumpy masses, and have fallen frequently in this form to the foot of the cliff, and are possibly what are described as “lumps of Kaipara limestone.” In some places the veins are beautifully crystalline, containing large crystals of calcite in dog-tooth spar form. At other times the calcite crystals are small (requiring a lens to distinguish their outline) but very numerous, and forming a white sparkling surface to the black grit.
The grit lies in all places conformably on the underlying sandstones, and is also overlain by sandstones in a per-
fectly conformable manner. The first outcrop, however, occurring at the end of the bay, is a most peculiar one. The grit thins out abruptly. This appearance is due, I believe, to currents, which have produced “contemporaneous erosion,” and is not an example of “current-bedding.”
The thickness is about 15ft., but apparently the bed thins out abruptly. This strange resemblance to an injected lava (there is, however, no alteration, of couise, of the surrounding beds) is frequently found in outcrops of the Cheltenham breccia, but not so frequently in those of the Parnell grit.
Sir A. Geikie, who gives a similar section, writes: “It shows a deposit of shale which, during the course of its formation, was eroded by a channel into which sand was carried, after which the deposit of fine mud recommenced.
It is evident that erosion took place, in a general sense, during the same period with the accumulation of the strata…. We may reasonably infer that erosion was due to the irregular and more violent action of the very currents by which the sediment of the successive strata was supplied.”
There is an example of “injection” in the Ponsonby tuff which cannot be explained by current-bedding, as will be seen later; but current-bedding is sufficient explanation of many of these unconformable appearances.
From Howick to Turanga Creek there is no outcrop of the grit along the cliffs. At Turanga Creek the greensands were found by me resting unconformably on Maitai slates, without any Papakura limestone between the two.
Along the cliffs. of the Manukau Harbour the beds, as a rule, are almost horizontal. In places, however, they have been inclined at high angles, and here and there much disturbed. It is generally at these spots of distortion that the volcanic beds crop out.
There is a wharf about a mile or a mile and a half from Onehunga. At low water more of the grit can be seen, as it forms the base of the cliffs between the fault and the White Bluff. Round the White Bluff the Parnell grit rises somewhat steeply; but that section has already been drawn and described in dealing with the question of the identity of the two breccias. The Parnell grit is here about 12 ft.—15 ft. thick, and it is not as coarse as in the eastern outcrops.
About two miles beyond the bluff the Parnell grit is again met with, and may be followed for some distance along the base of the cliffs round Cape Horn, where its hardness has been of great service in protecting the cliffs from the effects of marine denudation (Plate XXXVIII, fig. 6). The section shown in fig. 7 is taken about three miles from Little Muddy Creek, where the last section of the grit to the
west was observed. The dip of the sandstones near 2 is not actually seen. The grit here is about 8ft. thick. Lapilli however, are rare in it, and in some respects its general appearance does not resemble that of the grit. This may be due simply to the fact that it is a very westerly outcrop. On the other hand, it is possible that the bed seen here is a distinct one, possibly an outcrop of the Ponsonby tuff, to be presently described.
7. The Other Volcanic Beds.
Having dealt at length with the Cheltenham breccia and the Parnell grit, something must be said of the other volcanic beds. I have already said that I do not consider the coarse breccias containing Bryozoa as all identical beds, but find it impossible at present to distinguish between them. The finer tuffs of the same, or nearly the same, age may, however, be separated. Two of them are here described as the Wairau tuffs and the Ponsonby tuff. I should also mention that other beds contain occasional volcanic fragments, but not enough to constitute them tuffs. The Orakei greensand is a very good instance, for in that bed patches of scoriaceous lava the size of a pin's head may be detected with a lens. On the shores of the Tamaki, near Panmure, there are also pumice sands, but I believe these are Pliocene or later beds, unconformable to the Waitemata series, and derived from the pumice plateau in the centre of the North Island.
The Waitakerei breccias and conglomerates on the west coast and therefore on the other side of the vents, I have not visited; but they must, I believe, be classed with the Waitemata series.
It is interesting to compare the Wairau tuffs with that formed at the eruption of Tarawera. Except that the former were submarine, the resemblance is considerable; but, judging from their thickness and distance from the vent, the eruptions which produced them were larger. It is interesting to note that there are three or four tuffs separated in each case by a few inches of shale, so that the eruptions succeeded each other at only short intervals. These eruptions took place some time after those which produced the Cheltenham breccia, and were not, apparently, so violent.
I have given this name to volcanic beds which are best developed at the Wairau Creek, near Lake Takapuna. There are really several beds, each separated from the next by a thin layer of shale. The distinction between a tufaceous sandstone and a sandy tuff is not very easy to draw, and some of these beds are decidedly sandy. The lowest, however, is
distinctly volcanic, with, occasionally, fragments of scoria as large as a pea.
The position of the Wairau tuffs appears to be above the Cheltenham breccia and below the Orakei greensand. I am uncertain whether they are above or below the Ponsonby tuff, since they seem everywhere to be above it; but at Ponsonby, where the section is clear, either they are not above it or they must be more than 100 ft. above it, a most unusual position.
The group of tuffs is more sandy than the Parnell grit, but the scoria can easily be seen with a lens. At the Wairau Creek I discovered a fragmentary Gastropod shell near the base of the second bed, It is very minute, not as large as a pea, and several whorls are evidently missing; but I believe it is a species of Littorina. Elsewhere I have not been able to detect any fossils.
Since there are no fossils, it will be advisable to give the reasons for identifying volcanic beds at Howick, St. John's College, the Tamaki, and Manukau Harbour, with this bed at the Wairau. In the first place, the beds do not resemble the ordinary Waitemata sandstones, which are blue or grey, while these tuffs are brown or black, with a tendency sometimes to spheroidal weathering. In the second place, although there is a little scoria present occasionally in other beds, it is never so abundant. In these beds, too, occur veins of calcite, with large crystals sometimes ½ in. long (dog-tooth spar); and there are also veins of zeolites. And, moreover, these beds always occur as a well-defined group of black bands in the cliff, the lower being the more tufaceous.
At the north head of Castor-oil Bay the tuffs are separated by bands of sandstone, as at St. John's College. Generally they are separated by thinner layers of shale.
In the first section I observed the bed was about 8 ft. thick. Across Castor-oil Bay no section can be seen, the ground being low. In the floor of the bay nothing can be seen, owing to the covering of yellow sand. If there is no break—and there is no reason to suppose one—the Wairau tuffs are here about 30 ft. above the Cheltenham breccia.
To the north of the Wairau (Castor-oil Bay) I saw nothing of the tuffs, but some of the finer tuffs at Whangaparaoa resemble them.
On the southern side of the harbour there is no outcrop of the tuffs until St. John's College is reached, the reason being that younger beds occur along the cliffs. At St. John's College they are seen in the floor of the stream when the tide is out and a freshet has scoured away the mud. The bed of the stream is obscured farther up by a raupo swamp, and the hillsides are thickly covered with gorse. Wherever the strata can be seen they are, however, sandstones, so that it seems
fairly certain that the Wairau tuffs are here about 70 ft. below the Parnell grit, which coincides with the supposed general position of the beds. The tuffs are more sandy and thinner than at Wairau.
The Wairau tuffs occur at the Tamaki, near the west head. Apparently they are above the Orakei greensand, but faults make it impossible to say with certainty.
At Howick the Wairau tuffs again crop out, this time in numerous thick bands separated by several layers of shale, and shading of into sandstones. Here they are traversed by numerous zeolite veins. They occur not far from the last outcrop along Howick cliffs of the Parnell grit, but the relation between the two is obscured by several faults.
At Maungamaungaroa Creek there is another outcrop, and, judging from the dip, these beds should be nearly at the lowest horizon of the Turanga greensands, the Parnell grit occurring a little above the greensands.
As seen in a section, close to Onehunga, on the Manukau Harbour, the tuffs resemble the outcrop at Howick.
With regard to the Wairau tuffs, I am inclined to think that, just as in the case of the Cheltenham breccia, it is a group on the whole synchronous (and probably marking a gradual quiescence of the Waitakerei chain), but derived from different vents along the chain, just as in the former case.
At the Tamaki and at Maungamaungaroa Stream there occurs a thin tuff (2 ft. thick), with numerous small lapilh, traversed by veins of large calcite crystals. Its position is 30ft. below the Orakei greensand, as seen at the Tamaki Gulf, some distance from the west head. I have not seen it elsewhere.
The Ponsonby Tuff.
The Ponsonby tuff is a most interesting bed. In the first place, it is quite a thin bed, and yet has a wider distribution than any bed in the Waitemata series. In the second place, it shows some peculiar results of distortion. It is nowhere more than 2ft. thick, yet it occurs at the Tamaki, at St. Helier's Bay, at the Manukau, at Ponsonby, at Cheltenham, at Narrow Neck, at Wairau Creek, and at Deep Creek. When unearthed it is a blue soft bed, rather sandy to the touch, speckled with white flakes of kaolinised feldspar. These flakes are thickly crowded together, sometimes as many as 500 in a square inch. Occasionally a small red patch of scoria is seen. The rock weathers to a pale-yellow in which the white feldspars are still visible.
On the whole, the bed grows thicker and coarser in a north-westerly or westerly direction, and I believe it came
from some explosive outburst of a Waitakerei volcano. At the Tamaki it occurs as a thin bed 1 ft. thick not far from the west head. There is a bay and two faults at least between it and the Parnell grit. At St. Helier's Bay it occurs at Watson's Point, the westerly head of the bay, among very much contorted strata. It is not very far from the reef of the Cheltenham breccia. At the Manukau it occurs near the Wairau tuffs. Here it has been drawn out in a most curious fashion. The spot is near an area of distortion. All the lumps are elongated in the direction of the dip of the beds
At Cheltenham it occurs near the breccia, much crushed. At Wairau Creek it occurs on the bank of the creek in the most distorted part of the cliff, and has been crushed out completely, occurring as a lenticular mass.
In all of these localities it seems to be a little above the breccia.
Across the neck on the North Shore no section can be seen, but the dip continues regular. The tuff would therefore seem to be more above the breccia than is usually the case, and some fault may be present.
In the section at Acheron Point the relative positions of the beds are quite clear, but it is not so certain that there is present an extension of the Cheltenham breccia seven or eight miles away. The question of its position, at Acheron Point has already been dealt with in describing the Parnell grit.
I have now described all the volcanic beds of the Waitemata series, and (to recapitulate) have come to the following general conclusions: That in late Oligocene times, in the shallow sea near Auckland, there rose a long line of vents, which were at first very powerful and gave rise to several coarse breccias, laid down amid conflicting currents on the fossiliferous sea-floor. But, as usual, after these first violent eruptions, from which were derived the Cheltenham breccia and the Whangaparaoa breccia, and no doubt several others, besides the massive breccias found on the west coast, there were other manifestations of volcanic activity, on the whole more feeble, but some explosive eruptions on a large scale; and it is to these we must look for the source of the Wairau tuffs and the Ponsonby tuff. Meanwhile the whole area gradually sank, till, towards the close of the Miocene period, the Palaeozoic islands and ridges were mostly covered by the waves of the advancing sea, which laid down in regular succession shales and sandstones, even on the older lava-streams of the Waitakerei vents. But at the same time, and probably earlier too, volcanic outbursts were taking place on the Coromandel Peninsula, some of them of sufficient magnitude
to scatter their debris over the floor of the Auckland sea; and it is to these we owe the Parnell grit. At a later period the volcanic forces became quiescent, the Miocene strata were raised and extensively denuded, and between the two former lines of volcanic activity arose scores of puys, which covered the older strata with tuffs and basic lavas, and distorted the beds, raising them sometimes, perhaps, from the sea; though I believe the majority of these puy cones arose on the land. One conclusion of interest is that these areas of volcanic disturbance were areas of subsidence, not elevation.*
It is possible that the Waitakerei outbursts were at first submarine, and the opening phase may have been one of elevation. But while the vents discharged their contents the sea-floor gradually sank. It was only when volcanic energy had completely died away that the consolidated sandstones, shales, and tuffs were thrown into long gentle anticlines and synchnes, to be denuded by the atmospheric forces until, after another phase of volcanic history of quite a different type, they again began to sink beneath the waves.
[Footnote] * See Sir A. Geikie: Ancient Volcanoes of Great Britain,” vol.p. 470.