Art. XXXIX.—Pumice: Its Geological Distribution on the East Coast of the North Island of New Zealand, extending from Tologa Bay (38° 20′ S. lat.) to Cape Turnagain (40° 30′ S.).
[Read before the Hawke's Bay Philosophical Institute, 14th November, 1887.]
In a short paper which I had the honour of reading before this Society last year, on “Traces of Volcanic Dust-Showers at Napier, and generally throughout the East Coast District,”* I mentioned, incidentally, that I had been engaged for some time past in collecting data as to the age and the extent of the pumice deposits so common throughout the district. I now beg to submit the results of my investigations as far as I have yet gone.
Pumice is a substance essentially volcanic in its origin, and is always more or less common in volcanic districts. It belongs to the class of volcanic rocks known as vitreous, as distinguished from crystalline, and to the sub-class acidic, as distinguished from basic. Rocks are said to be acidic when they contain as part of their constituent ingredients more than 60 per cent. of silica. When they contain less than 60 per cent. of silica
[Footnote] * “Trans. N.Z. Inst.,” vol. xix., p. 385.
they are said to be basic. According to Jukes, the following substances enter into the composition of pumice. The maximum, minimum, and mean percentage of each substance is given:—
|Substance.||Maximum percentage.||Minimum percentage.||Mean percentage.|
|Oxide of Iron and Manganese||4.5||0.5||3.2|
From this list of constituents it will be seen that, notwithstanding the generally supposed sterility of pumice, it contains in its composition every essential product needful for the making of what is known as a fertile soil. Indeed, I have often thought that a good deal might be said in support of the claims of pumice as being the origin, in this district at least, of most of the fertile soils. By far the greater portion of a mass of pumice is composed of silica and alumina, which, in a state of chemical combination, form clay. The same mass contains soda and magnesia, which form a part of every rich soil; and the same mass also contains potash, which is essential to the fertility of all soil. Liebig, in his “Chemistry of Agriculture and Physiology,” says that a Hessian acre of disintegrated felspar 20 inches deep contains 1,152,000 lbs. of potash. This would amount to 1,120,000 lbs. in an English acre 12 inches deep. Now in pumice there is about one-fourth the quantity of potash as there is in the same weight of felspar, so that in an acre of disintegrated or decomposed pumice 12 inches deep there must be something like a quarter of a million pounds of potash, whilst the quantity of soda in the same disintegrated mass would weigh nearly 400,000 lbs. The percentage composition of an orthoclase felspar, according to Rutley, is S1 O2 = 64.20, Al2 O3 = 18.40, K2 O = 16.95. I am not aware of any special analytical tests having been made in New Zealand as to the composition of pumice, but I am inclined to the opinion that the pumice found throughout the East Coast of this Island contains less silica and more alumina than the amounts stated in the table quoted above.
Pumice, such as is to be met with in large quantities along the sea-beach of Hawke's Bay, is a light spongy-looking kind of rock which swims upon the surface of water. In reality, pumice is a porous or vesicular glass, closely allied in its composition to obsidian, and, in fact, it is found on the surface of obsidian lavaflows in volcanic districts generally. It must, not, however, be supposed that pumice is always to be found on the surface of
lava-flows, or that it is only produced on the surface of such flows. There is sufficient evidence in the volcanic district of this island to show that such is not the case.
Pumice is usually of a dull grey, varying to a pale straw colour, although there are many places, both within and without the volcanic zone, where the pumice varies from a deep red to a pale salmon colour. The varieties, however, both in colour and composition are legion; in fact, a scale might be made of varieties of pumice, passing gradually from a beautiful white flourlike pumice, through coarse sand grit and boulder pumice, to a variety much resembling a trachyte, and thence to a kind of cross between a pumice and an obsidian. The numerous specimens on the table will fully illustrate this statement. Obsidian froth would perhaps best describe the pumice pebbles, such as are brought down from the pumice fields on the Taupo Plain by the Ngaruroro, Mohaka, and other rivers in time of flood, the composition of pumice and obsidian being almost identical. Although pumice appears to be so very light, its specific gravity is from two to two and a half times the weight of water. In other words, a cubic foot of pumice would weigh from 2,000 to 2,500 ounces. The beautiful specimen of obsidian which was found lately at the mouth of the Turanganui River, and which I have brought down for inspection by members, will give some idea as to the actual specific gravity of an apparently light substance like pumice.
As far as my own observations extend, no volcanic product is more widely diffused over the North Island of New Zealand than pumice and trachyte-pumice, which is a pumice containing small felspar crystals within its mass. The surface of the country within a radius of thirty miles or more of Lake Taupo is composed of nothing but pumice, underlaid for the most part by trachytic-lava rocks. Were I dealing with the character of the rocks found in the volcanic zone, as I hope to do at some future time, I should endeavour to account for the origin of the vast pumice deposits of that district; but my purpose in the present paper is to show how far the country to the eastward of the volcanic zone, and extending from the great central mountain chain forming the Ruahine, Kaimanawa, and Te Whiti Ranges to the sea, on the one hand, and from Tologa Bay (latitude 38° 20′ S.) to Cape Turnagain (latitude 40° 30′ S.), on the other, has been affected directly and indirectly by volcanic products in the shape of pumice and other ejectamenta. My second object is to trace the geological distribution of this important product.
The district named above may be said to contain an area of 5,000 square miles, and, as far as I am aware, there are only three places in the whole of this area where traces of igneous rocks are to be met with. These places are (1st) at
Tarawera, midway between Napier and Taupo, where the same trachytic-lava rocks are to be seen as are met with throughout the whole of the central volcanic district extending from the Paeroa Mountains to Runanga, a place situated on the Tarawera-Taupo Road. At the latter place the trachytic-lavas are seen to form the bed of the Waipunga Stream; (2nd) at Red Island, some miles along the coast to the south of Cape Kidnappers, where Mr. McKay (“Geological Reports, 1874–76,” p. 45), says igneous crystalline rocks occur; (3rd) at Woodville, in the Seventy-mile Bush, where a hard compact reddish rock, with a cellular and scoriaceous surface-structure is met with in a small creek which runs through the grounds adjoining the public school. The rock appears to be a variety of trachyte. Possibly other igneous rocks will be found as the country south by east and south-east of Tarawera becomes better known, where I imagine is the termination of the great lava stream which at one time occupied a large portion of the valley through which the Waipunga Stream and its numerous tributaries now flow. Indeed, I should not be surprised if the trachyte-lava rock is eventually found underlying the lowest beds of blue-clay marls which are to be met with in some places flanking the east slopes of the central mountain range, as in the Tukituki and Ngaruroro Rivers. But though the evidence of the existence of volcanic rocks is so very limited—and there are no traces whatever of eruptive vitreous rocks except at Tarawera and Woodville—the whole country bears the evidence of having been greatly modified even in its surface-features by volcanic products known as ejectamenta.
The geology of the district under notice has been in great part already described by the Geological Survey Department; Mr. Percy Smith, Assistant Surveyor-General, has also published a report in the “Transactions,” vol. ix., p. 565, on the “Geology of the District between Napier and Mohaka.” The results, though necessarily imperfect and incomplete, show that the district, though offering examples of Mesozoic and even of Palæozoic rocks, is essentially a Tertiary and Post-tertiary one, and offers ample testimony of vast movements, which have been brought about mainly by the action of water. According to the geological map published with the “New Zealand Handbook” by Dr. (now Sir James) Hector, the Director of the Geological Survey, a large proportion of the rocks exposed along the coast between Tologa Bay and Cape Turnagain belong to what are known in this country as the Cretaceo-tertiary formation. The older Tertiaries, known as Eocene and Miocene, are shown on the map to lie immediately to the west of the so-called Cretaceotertiaries, both to the south and north of Napier. Between Tiwhinui, a few miles to the south of the Mohaka River mouth, and Cape Kidnapper in the Hawke's Bay river system, the
surface rocks belong to the later Tertiary or Pliocene and the Post-tertiary formations. These later deposits cover an enormous area inland, extending from Napier in a north-west direction beyond Pohui, on the Taupo Road, and in a west and south-west direction through the Heretaunga and Ruataniwha Plains, and thence onward and through the Seventy-mile Bush as far as the Manawatu Gorge. Similar rocks cover a large extent of country in the Poverty Bay District, and extend inland from Gisborne in a south-west direction as far as Te Kapu, near Wairoa or Clyde, although not so shown on the geological map. From specimens of fossils lately received from Mr. Balfour, sheep-farmer, who resides in the middle basin of the Mohaka River, it would seem that similar young rocks are largely developed in that district also; and hence a continuous belt of later Tertiary and Post-tertiary deposits can be traced through the East Coast District for a distance of nearly 200 miles. Westward, beyond the younger Tertiaries and Posttertiaries, lie the Ruahine and other mountains of the central chain, the rocks of which belong to the older Mesozoic and Palæozoic formations. Now it is a curious circumstance that over by far the larger portion of the district under notice traces of pumice are to be met with, either upon the surface or immediately below the dark soil, which in a former paper I referred to as being partly of volcanic origin, and which covers a large part of the country in the Hawke's Bay District. This pumice is found as a surface-pumice equally upon the summit of the Ruahine Mountains as upon the Napier Hills, or upon the hills extending between Napier and Tologa Bay. Neither the surface irregularities nor the rock formations have made any difference in the distribution of the pumice over such a wide area. On the top of the Ruahine Mountains the pumice is in the form of large pebbles, such as are now to be found in immense deposits on the Kaingaroa Plain, in the Taupo District, and similar to those which are brought down and deposited along portions of the Napier beach by the Ngaruroro and other rivers. Much of the pumice underlying the surface-soil is of the character of grit, or coarse pumice-sand, although in many places the pumice is as fine as the finest flour, and as white as snow.
As far as I can gather, there are three very clearly defined periods of pumice deposition. These are:—
A surface or subsoil deposit of comparatively recent age, and referred to above as covering most of the country to the west, north-west, and north of Napier;
A Pliocene pumice deposit of vast extent and thickness, interbedded with shingle, clays, lignite, and fossiliferous sands;
A Cretaceo-tertiary? (Miocene) pumice deposit, found interbedded with the blue-clay marls.
First period.—Recent.—Surface or subsoil deposit.
As remarked above, this is a widely-spread deposit, covering the surface of the country over a large area. It can be traced continuously from Napier to Tarawera, and from Gisborne to Tarawera. At Napier and Gisborne it is a subsoil pumice, whilst at Tarawera it is a surface pumice. At Napier this pumice is seen to overlie pumiceous clays, and these in turn overlie pumice, scoriæ, and in places grit beds. These latter beds do not appear to belong to the higher pumice beds, which can be traced back as belonging to the surface pumice deposits met with at Tarawera, and thence onward to the Taupo Plain. I do not see how to account for the existence of this loose, coarse, sandy pumice, except on the supposition that at some period towards the close of the Post-tertiary deposits vast showers of pumice took place, which were succeeded by showers of volcanic dust not unlike those which took place at Tarawera in July last year. The small rounded pumice grits, which become larger and coarser as we proceed in the direction of the volcanic zone, show, by their shape, that they must have been subject to a large amount of attrition; and their peculiar position in the hill-sides goes to show that water had nothing whatever to do in their deposition. The entire district between Napier and Poverty Bay is more or less covered with this characteristic deposit, which is overlaid by a very thin coating of dark black soil.
Second period.—Pliocene.—A pumice deposit, associated with shingle, conglomerates, sands, blue clays, and lignite.
This is a very remarkable deposit, and characterises the period as one of vast movements brought about mainly by the action of fresh water. The beds forming the southern shore of Hawke's Bay, and extending from what is locally known as the Black Reef, 2 miles or so within Cape Kidnappers, to Havelock, and thence onward to Pakipaki, are the typical representation of the pumice deposits of this period. The beds composing this very interesting section must be, at the very least, from 1,500 to 2,000 feet in thickness, for they extend in a most regular manner for several miles as high cliffs, dipping to the W.N.W. at an angle of about six degrees. The pumice alternates in these beds with shingle, clays, blue and straw-coloured sands, conglomerate and lignite, and, including the different beds, the pumice must amount to several hundred feet in thickness. The pumice varies in character in the different beds, from pebbles two or more inches in diameter, to a very fine deposit somewhat resembling chalk, and of a beautiful whiteness. In some of the pumice beds fine specimens of fossil leaves are met with, and these are as perfect in structure as if just gathered from a tree, their black shiny carbonized surfaces being the only distinction between them and freshly-gathered leaves. Every vein and veinlet are seen, but their beauty soon fades when their surfaces are exposed to
the air. Many specimens of fossil leaves are also to be met with in the lignite beds, which are found in the lower part of the section. With the kindly help of Mr. Hamilton, our Secretary, I have been able to obtain some fair drawings of a few of the leaves found by me, and also of some from the pumice beds of the Poverty Bay District.
The pumice deposits, of what I have ventured to name the Kidnapper section, pass underneath the Heretaunga Plain, and they re-appear again at the back of Maraekakaho. At Redcliffe, near Taradale, six miles from Napier, the Kidnapper beds are largely developed, the pumice beds being underlaid and overlaid by conglomerate beds of a deep-red colour. These pumice and conglomerate beds are seen to dip to the E. by S. underneath the bed of the Tutaekuri River, in the direction of the Kidnappers. In the Redcliffe pumice deposits a ruby-kind of quartzite is found embedded here and there, and some mica scales are met with at the junction of the pumice and conglomerate beds. From Maraekakaho, both north, west, and south-west, the pumice deposits have an enormous development. In the direction of Hampden and the Ruataniwha Plain they cover the entire district, all the hills being composed of pumice and shingle, alternated here and there with clays and an impure lignite bed. The hills forming the watershed between Maraekakaho and Hampden, and which run southward along the east side of the Ruataniwha Plain, opposite the Guavas station and Hampden, and forming the left bank of the Manga-o-nuku Stream, are composed of pumice, shingle, clays, blue sands having a tinge of green, and lignite. These deposits correspond to the Kidnapper middle and lower beds. These pumice beds are seen to pass underneath the Ruataniwha Plain near the Guavas station, and they are again met with in the Tukituki River, on the west side of the plain, just below the crossing leading from Te Onga-onga to Makaretu. The beds at this point are dipping to the south-east. Six miles further up the river the pumice beds are again exposed, dipping south-east, and having their strike in the direction of Ashley-Clinton. At this place the pumice and conglomerate beds form the tops of the highest hills in the district. The pumice is fine in texture, but is underlaid by coarser pumice-sands and pebbles, intermixed with angular sandstone grit. About nine miles further to the south by west, is the Scandinavian settlement known as Norsewood. The height of this township is something like 1,300 feet above sea-level, and until taken up for special settlement purposes the district formed a part of the Seventy-mile Bush. Here shingle lies immediately underneath a poor soil, and below the shingle come the characteristic conglomerate and pumice beds of the Kidnapper section. The lignite bed is also met with, it being exposed in most of the creeks in the whole
of the bush district. In one exposure, between Norsewood and the hills which flank the Ruahine Mountains, the lignite is exposed for a thickness of 20 feet, but it is perfectly useless for domestic purposes. Between Norsewood and Danneverke the same pumice and attendant beds are to be met with in every exposure by the wayside, and it would appear as if the whole of the district,—from the lower hills flanking the spurs of the Ruahine mountains to the limestones and marls which pass down the left bank of the Manawatu River,—was simply an extension of the Kidnapper pumice section. I have followed portions of these pumice beds as far as the Manawatu Gorge, where the shingle conglomerate and pumice are met with overlying limestones, as is the case in so many other places. The pumice deposits to the west, north-west, and north of Marae-kakaho all belong to the Kidnapper section. They can be traced from Maraekakaho to Kereru, thence to Pohui, Te Kapu, Murumuru, Te Reinga, and onwards to Poverty Bay, where they form the lower slopes of all the hills to the east, north, and north-west of Gisborne township. In the Poverty Bay pumice beds the shingle is not so largely developed as in the district further south, but fine specimens of fossil leaves are obtainable from the white pumice beds. The leaves are quite unlike those found in the Kidnapper beds already referred to. They are of smaller size, and most of them appear to be similar to leaves which may now be found growing in the higher and more elevated lands of this island.
Up to the present time I have found 44 varieties of fossil leaves in the Poverty Bay beds, and two specimens of lycopods. In the lignite beds of this district fossil ferns are abundant; but I have not yet been able to spare time to make a collection, such as I hope to do shortly. In the “New Zealand Guide to the Geological Exhibits at the Indian and Colonial Exhibition,” published last year, Sir James Hector, in his classification of the rocks constituting the Pliocene formation, places the pumice and conglomerate beds just described below what are known as the Napier or Scinde Island limestones. The classification made by the Geological Department of the Pliocene rocks, in descending order, is as follows:—
Napier series (limestones and marls);
These several series are followed by the Te Aute series, which the Geological Department places at the top, or among the highest beds of the Miocene formation. In the “New Zealand Handbook,” also published by Sir James Hector, page 26, the classification of the Pliocene formation is somewhat different from the above, the Scinde Island limestones being placed with
Terrace plains, as the youngest of the Pliocene series. According to this classification, the pumice deposits of what I have termed the second period of deposition, as described above, are considered as being older—indeed, much older—than the Napier limestones and marls. The official classification has evidently been based upon the report furnished by Mr. McKay, the Assistant Geologist, who, in his report on the “Geology of the District between Napier and Waipukurau,”* says: “Lying in the low grounds to the west of Shrimpton's, and forming the strata underneath the river-gravels of the Ngaruroro Plains, is a great development of sands and gravels with beds of clear white pumice, sand. All these strata dip in an easterly direction, and apparently pass underneath the marly strata and Scinde Island beds forming the hills to the east.” For my part. I do not see my way to accept the classification of the Geological Department in relation to the pumice beds and the Scinde Island limestones; in fact, the evidence to me is overwhelming in favour of the following classification:—
Pliocene formation, in descending order,—
Pumice, conglomerates, blue clays, and lignites;
Kereru Rotella crags and fossiliferous sands;
Scinde Island lower limestones, the upper limestones being the equivalents of the Kereru crags.
So as to test the question fairly, every section for many miles round about Napier has been gone over by me, and I have followed the pumice beds of the Kidnapper section past Havelock, Pakipaki, Maraekakaho, thence south-west up the Maraekakaho Stream, and down the Manga-o-nuku Stream as far as the Waipawa-Hampden Bridge; and in no single instance is limestone of any kind to be found overlying pumice beds, but there are many examples of pumice and conglomerates overlying limestones. I have also traced the Kidnapper pumice beds in a N.N.W. direction as far as Pohui, thence N.E. through a large portion of the Cook County; and in no single instance in this direction have I seen pumice, or any of the beds representing the Kidnapper pumice section, underlying limestone. Everywhere the evidence shows pumice, shingle, clays, etc., overlying limestones—as for example, at Napier, at the Black Reef in the Kidnapper section, at Petane, Tongoio, Roy's Hill, Pakipaki, Maraekakaho, Te Reinga, Poverty Bay, and many other places. In one of the Geological Reports issued by the Department, it is stated that pumice is to be found inter-bedded with the Scinde Island limestones, and that pumice is met with at Puketapu underlying the limestones. I have not yet been able to find either of the beds referred to, or even traces of them.
[Footnote] * “Geological Reports,” 1879.
On the Napier Hills, lying unconformably on the upper limestones or the marls, large deposits of pumice, pumiceous clays, scoriæ, and in some places grits, are to be seen. At the Breakwater Point the pumiceous deposits must be quite 35 feet in thickness, and on the north-east side of the Napier Hills the beds are even of greater thickness. At Puketapu, where pumice is said to underlie limestone, there is a large development of conglomerate and pumice; and the beds composed of these materials can be well seen resting on the Napier marls on the roadside near Alexander's, on the Puketapu-Petane Road; whilst a little further on, in the direction of Puketapu, the limestones (Napier upper) are met with overlying the same marls; that is, on the denuded surface of the marls are to be seen pumice and shingle, whilst on the undenuded marls rest the limestones. Now, are the Napier lower pumice and pumiceous clay deposits and the Puketapu shingle and pumice deposits to be classed as belonging to a period much later than the pumice beds of the Kidnapper section, or are they to be classed as belonging to the Kidnapper section? If to the latter, then it is clear that the classification of the Geological Department is a wrong one; and if to the former, then it must follow that the Napier limestones are simply a part of the Kidnapper pumice section, having pumice, shingle, and conglomerates below them, and pumice and conglomerates above them; but, as far as I can find, there is not a particle of evidence in support of such a classification. If the Kidnapper pumice section is older, as the Geological Department say it is, than the Napier limestones, it follows that the Napier pumiceous clay beds belong to a period corresponding in point of time to the “dispersed gravel series,” or to a more recent period still—that is, to Post-tertiary times. But such an arrangement cannot be supported by a tittle of evidence. As already remarked, every section and every exposure, as far as I have any acquaintance of the district, supply facts showing the pumice, gravels, sands, conglomerates, and lignites of the Kidnapper section as overlying the Scinde Island limestones, and that the Napier pumice beds form a part of that section. In contending here for the greater age of the Scinde Island limestones, as compared with the pumice deposits, I do not desire it to be understood that the limestones upon which the pumice and included beds are seen to rest are all of the same age, for the contrary is the fact. My contention is simply that the Kidnapper pumice section, which is so largely represented throughout the East Coast District, is not below but above the Scinde Island limestone; and that the pumice deposits, as represented by the Kidnapper, Scinde Island, and other beds, are the youngest of the Pliocene series, with the exception of “dispersed gravels.”
The height of the pumice deposits in the localities named above varies considerably. In the Kidnapper section itself the
highest beds are nearly 400 feet above sea-level. In Napier the highest beds exceed 300 feet in height; whilst at Redcliffe, near Taradale, the highest pumice bed met with is not more than 150 feet above sea-level. Behind Maraekakaho, and midway between it and Hampden, the pumice is found at a height of about 1,000 feet, and this height is maintained along the eastern and western sides of the Ruataniwha Plain, and thence onward to Norsewood. The pumice and shingle deposits in the Poverty Bay District, in the immediate vicinity of Gisborne, are not less than 750 feet in height. It would seem as if the area now forming the Ruataniwha, Heretaunga, and Poverty Bay plains—and, indeed, the entire country eastward of the great axial ranges of the island, was, towards the close of the Pliocene period, simply one vast surface-deposit of pumice, shingle, lignite, and blue (volcanic) clays, of great depth; that the sea-shore was much farther to the eastward than it now is; that the beds of the rivers flowing at that time from the west were hundreds of feet higher than they now are; and that aërial and aqueous agencies, operating throughout a long period of time, have brought about the lowering of river-beds, the formation of Post-pliocene plains and valleys, and the disappearance of a large area of land to the east of the present coast-line. Of the changes produced on the land, as indicated above, the facts are patent to every observer, and that the land was once greatly extended eastward of the present coast-line must also be evident to those who, like myself, have travelled for any length of time along the coast. Wherever the blue-clay marls are exposed, denudation proceeds at a surprisingly rapid rate. In some places whole hill-sides are now moving seawards—as at Waimarama, Mohaka, Poverty Bay, and other places further north; and the numerous extended reefs which are exposed almost everywhere along the coast at low water show that this process of coast denudation has been going on for a long time past.
I do not know whether the time is near or distant when geologists will be able to study the geology of places along the same parallels of latitude both to the north and to the south of the equator; but it seems to me that a good deal might be learnt of the physical and cosmical earth-changes from this mode of study. In concluding my account of the second period of pumice distribution in this district, I venture to make an extract from Charles Darwin's description of the northern portion of Patagonia, as given in the “Voyage of the Beagle.” Mr. Darwin's description of the Pliocene rocks of Northern Patagonia, in the corresponding latitude as the Pliocene deposits of Hawke's Bay, agrees in a striking manner with the deposits as found in this district—indeed, the description of certain beds found by him in his exploration of Northern Patagonia might almost be used
to describe some of the beds belonging to what I have described as the Kidnapper pumice section. His words are:—“These beds are covered by others of a peculiar white stone, including much gypsum, and resembling chalk, but really of a pumiceous nature. These white beds are everywhere capped by a mass of gravel, forming probably one of the largest beds of shingle in the world.” … “Yet all this gravel has been transported, and probably rounded, subsequently to the deposition of the white beds, and long subsequently to the underlying beds with the tertiary shells.”
I ought to have mentioned that the Moa footprints which may be found at the mouth of the Turanganui River at Poverty Bay, at low water, are in the pumice deposits, which are largely exposed at this place. Footprints of birds were also found in the pumice beds on the Napier Hills, when the fresh water reservoir was being dug out at the corner of Lighthouse Road.
Third period.—Cretaceo-tertiary? (Miocene).—A pumice deposit, interbedded with blue marls.
This, I suppose, must be called the Cretaceo-tertiary pumice deposit, because in each place, with a single exception, where the pumice is found the rocks are classed as Cretaceo-tertiary by the Geological Department. No mention is made, as far as I know, in any of the geological reports of the existence of a pumice deposit among the Cretaceo-tertiary rocks; but there can be no doubt as to the existence of such—that is, if the classification made by the Geological Department be a correct one. Commencing at Tologa Bay, at the most northerly point of the district dealt with, the blue marls and clays running along the northern shore of the bay are described (in “Geological Report, 1887,” page 152), as belonging to the Cretaceo-tertiary formation. The cliffs at this place rise to a height of about 150 feet, and they dip to the north-west at an angle of about five degrees. Interbedded with the blue clays and marls is a band of somewhat coarse pumice-sand, which varies in thickness from 2 feet to 3 feet. Overlying the pumice band the rocks appear to have scattered through their mass large pumice grits, as if the clays and pumice-sands had been mixed together by the action of water. This is characteristic of many of the blue rocks which are exposed between Tologa Bay and the Mahia Peninsula. In some places the pumice is of the size of large pebbles. As regards the pumice band itself, it is continuous throughout the whole extent of the exposed beds. About 10 miles to the south of Tologa Bay there is a place on the coast known as “Hole in the Wall.” It is so called from a small archway formed in the rocks by sea-action, and subsequently enlarged as a roadway to avoid a somewhat dangerous beach. Here a pumice band again appears, similar to that seen at Tologa Bay. The rocks dip to
the N.E., and the pumice band, which is nearly 3 feet in thickness, is quite conformable to the underlying and overlying blue marls and clays. In this pumice band I found a single ovoid pebble 5 inches long, 3 inches wide, and about 1 ½ thick. It is evidently a volcanic bomb. In the Geological Report referred to above, it is stated that at the “Hole in the Wall” are calcareous marls, between which is a “grit bed forming the roof of the archway.” The grit referred to is pure pumice, and could not be distinguished from bedded pumice in the Taupo District. The rocks at this place are classed by the Geological Department as belonging to the Cretaceo-tertiary formation. On the south side of Poverty Bay is the cape known as Young Nick's Head—the first land named in New Zealand by Captain Cook. The rocks forming the cape are identical, both in character and fossils, to the Tologa Bay beds, except that, instead of one pumice band, two pumice bands are met with, the upper one being less pure than the lower one. The rock immediately overlying the upper band is very fossiliferous, Foraminifera being abundant. At the mouth of the Mohaka River, which empties itself into Hawke's Bay to the north of Napier, the blue-clay marls are largely exposed. These marls are also classed by the Geological Department as Cretaceo-tertiary. Interbedded with the marls is a pumice band of from 10 to 15 inches in thickness, and it is constant wherever exposed. This pumice band is of the same colour as the overlying beds, and on its upper surface numerous fossil Foraminifera are met with. The rocks at this place dip to the south-east at an angle of about eight degrees. Cape Kidnappers, which forms the southern boundary of Hawke's Bay, is shown on the geological map as belonging to the Cretaceo-tertiary formation; and a similar pumice band of about 15 inches is seen exposed near to the most eastern point of the cape. It is interbedded conformably with the underlying marls, and dips to the west by north at an angle of about five degrees. Further evidence of the general diffusion of a pumice band through so-called Cretaceo-tertiary rocks is to be found inland between Poverty Bay and Wairoa, at Ahi-manu, Te Kapu, and other places. The pumice bands, three in number at the former place, are at least 1,200 feet above the sea-level. At Porongahau, also in the direction of Cape Turnagain, the pumice band is met with in similar rocks to those exposed in so many places along the coast. Of the general diffusion of a pumice band among rocks of a much older date than the later Pliocene there can be no doubt whatever, as the foregoing facts go to prove; but I am of the opinion that these so-called Cretaceo-tertiary rocks are younger than the official classification would make them. For my part, I am inclined to place them intermediate between the Eocene and Miocene, and I think
the fossils found in the series will bear out this classification. How the pumice band came to be deposited among marl-clays over such a vast area, and quite conformably to the underlying and overlying beds, I am at a loss to explain. I venture, however, to offer a suggestion. Are the blue-clays so common throughout the district simply volcanic ejectamenta, which have been subsequently acted on by aqueous agencies? I am inclined to think so. The blue-grey mud ejected from Rotomahana Lake at the time of the Tarawera eruption, last year, bears a close resemblance to the blue marl-clays in which the pumice band is found. Those who have been over the volcanic district of this island will readily understand that the eruption of mud from Rotomahana is only one of many similar eruptions which have taken place. Hot-water action and steam have played in the past, as they still play, an important part in what is understood as volcanic phenomena. The mixture of clay and pumice-grit and pebbles, of which such a large proportion of the rocks between Mahia and Tologa Bay are seen to be composed, suggests an inquiry as to how a mixture of this sort came about. Clay, equally with pumice, must be set down as a volcanic product; and I see no way to account for a conformity between the pumice and clays, except on some such supposition as stated above. The moya of the Central and South Amerian volcanoes is the mud of the Tarawera eruption; and we may suppose that eruptions of mud have been no uncommon event in the history of volcanic phenomena in this island. At least, such is my opinion, and such is the suggestion I offer, as accounting for the pumice band found in the rocks of this East Coast District, and which are classed by the Geological Department as belonging to the Cretaceo-tertiary formation.
The accompanying map (Plate XVIII.) contains all the places mentioned in this paper.