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.