Go to National Library of New Zealand Te Puna Mātauranga o Aotearoa
Volume 26, 1893
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Plates XLIX.-L.

On the Dyer's Pass Road, over the Port Hills to Governor's Bay, there occur several dykes. One of them (Pl. L., a), just above the Convalescent Home, runs north-west and south-east. It has been quarried for some distance, and is about 6ft. broad, and nearly vertical. It is probably classified as an andesite, but the deep weathering has rendered an accurate determination difficult. It seems to be composed of an interlacing network of feldspar microliths and augite grains, in which are porphyritic crystals of feldspar. This is probably labradorite, since the extinction-angles between twin lamellæ are greater than 37°, but not greater than 63°; also, a crystal in a section which shows no twinning, and is therefore probably parallel to the brachypinacoid, gave an optic axis out of the field, and a revolving axial shadow. There is no other porphyritic mineral. Probably augite is present, but only one large crystal was noticed, and that was greatly altered. The feldspar of the ground-mass is probably oligoclase, as the extinction is nearly parallel to the length of the microliths, and never greater than 5°; but the determination from the extinction of twin lamellæ did not give satisfactory results on account of undulose extinction. The augite of the ground-mass is greatly altered, forming brown grains of limonite.

Higher up on the road, on the south side, there is another dyke (b), running also north-west and south-east. It has not been quarried, and so it was very difficult to get satisfactory sections. It is about 10ft. wide, and shows a rudely columnar structure perpendicular to the sides of the dyke. The ends of the columns are easily seen, as on the lower side it appears as a wall about 12ft. high. This is probably basaltic, as sections show olivine to be present, though in all I obtained it is altered to limonite.

Below this dyke occurs an old quarry in a corner of a small gully (c). This has not been worked for some time, but quantities of stone lie about. This has been derived from a large dyke about 15ft. broad. The dyke runs generally in a north-westerly direction, but where the quarry occurs it has been bent, so that the two lines make an angle of about 15°. The dip of the eastern branch is about 75°, while the western is

Picture icon

Scale about 200 yds to an Inch. To illustrate paper by MR. Speight

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nearly vertical. There are other outcrops up the western side of the gully, and at a road on the other side of the spur (d), and again on an adjoining spur (e), where it has been quarried. The country rock is an olivine-andesite, but in the neighbourhood beds of laterite occur, produced by the weathering of the andesitic and basaltic flows, and by the subsequent pouring over them of hot lava streams. In no place were the beds observed to overlie the dyke, so it is most probably of later origin. The country rock has been altered near the edge of the dyke into a black rock in which crystals of feldspar appear, while a little further away it has become a laterite.

The dyke is composed of a dark-grey rock on fresh exposure, but near the edge it becomes black. It is more crystal-line in the middle, but contains vesicles throughout. These are not filled with infiltration products. Near the edge occurs a band of steam-holes, with their long diameters parallel to the edge of the dyke. There is not any well-marked columnar structure such as occurs in the dyke above.