Art. XXXVII.—On the Geology of the North Head of Manukau Harbour.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 2, 1869, Page 161

Art. XXXVII.—On the Geology of the North Head of Manukau Harbour. By Captain F. W. Hutton, F. G. S. (With Illustrations.) [Read before the Auckland Institute, August 16, 1869.] The cliffs, north of Manukau Harbour, are composed of a coarse volcanic agglomerate of various kinds of dolerite, trachyte, and rhyolite, the trachytic rocks, however, being much the most numerous. This agglomerate is generally horizontal, but at Paratutai—the rock that forms the north head of the harbour—it is seen to dip strongly to the north, and on its upturned edge rests a thin bed of vescicular doleritic lava, covered, conformably, with beds of agglomerate, to which succeeds another lava stream, also covered by agglomerate. The conformability of these doleritic rocks with the rest of the formation, together with their vesciular character, makes me class them as lava streams, instead of dykes. Proceeding northwards along the coast, several dykes of doleritic-trachyte, very similar to the lava streams already mentioned, but more compact, are seen, cutting through the agglomerate. The second of these, or the one first seen after passing the valley that divides the hill, on which the old pilot station was placed, from the rest of the cliffs, is about six feet thick at the base, and dips 80 S. E., but rapidly thins out upwards, and comes to an end less than two-thirds of the way up the cliff, showing that it has been injected from below, but had never reached the surface. Besides these true dykes there are also other reefs of rock which at first sight look like dykes, but, on a closer examination, are seen to be fissures filled up with fine-grained tufa, of the same composition as the matrix of the surrounding agglomerate. These fissures were perhaps caused by earthquakes at the time when the volcanic forces were in activity, and may help us to understand the original formation of some of the lodes at the Thames. Proceeding further northwards, at a distance of about a mile and a half from Paratutai, we find that the lower part of the formation has been thrown up by a fault, and is seen to rest upon beds of fine-grained tufa, tufaceous sandstone, and sandstone, which, no doubt, belong to the upper part of the Waitemata series; for similar rocks occur at Puponga, as described by Dr. Hochstetter. In the hill, under the old pilot station, a large angular mass of fine–grained tufaceous sandstone, interstratified with beds of shale, is seen, enclosed in the agglomerate. (Pl. IXa. Fig. I.) This mass is about twenty-five by fifty feet, and probably weighs-not less than 2000 tons; it belongs to the underlying Waitemata beds, and must have been thrown up by a volcano. That this volcano must have existed in the close neighbourhood, is proved by the large size of the block, as well as by the lava streams at Paratutai, although no trace of it can be now recognised; and the fact that the block, although composed of fragile materials, was not shattered in pieces, proves that it was ejected under water. On the eastern, or inner, side of Paratutai the cliff is being undermined and worn away at low-water mark (Fig. II.), while at high–water mark, or a little above it, another, and older, undermining of the sea can be observed, forming a terrace, the difference of height between the two being about ten feet, showing that the land has here risen that distance since the higher one was formed. This closely corresponds to the height of the raised beach at the Thames, on which Shortland and Grahamstown are built. On the outer, or west, side of Paratutai, a similar terracing exists, as can be seen in the Rev. J. Kinder's photograph; but I was not able to measure it, and so am unable to say whether the two are at equal heights.