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.