Go to National Library of New Zealand Te Puna Mātauranga o Aotearoa
Volume 34, 1901
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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-

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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

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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

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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.