Art. XLV.—Note on the Geology of the Country about Lyell.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 22, 1889, Page 387

Art. XLV.—Note on the Geology of the Country about Lyell. By Professor F. W. Hutton, F.G.S. [Read before the Philosophical Institute of Canterbury, 6th June, 1889.] From the first geological map of the Provincial District of Nelson (1864), published in the “Geology” of “The Voyage of the ‘Novara’,” from observations made by the late Sir Julius von Haast, up to the last (1886), issued by the Geological Survey

of New Zealand, the rocks in the neighbourhood of Lyell are shown as foliated schists and granite belonging to the oldest rock-systems of New Zealand, and forming part of a continuous band of those rocks extending from Separation Point down to and beyond the Teremakau River. Mr. A. McKay, however, in his report on the Reefton district,* “Reports of Geological Explorations,” 1882, p. 101. mentions incidentally that the rocks seen in the gorge of the Buller at and above Lyell are schistose rocks of unknown age, the principal part of which he believes to be Silurian, “although it is more than possible that much younger rocks are present.” I was therefore surprised, when I visited that district in January, 1887, to find it formed of sandstones and slates similar to those which have for several years been considered as belonging to the Maitai system, largely injected with granite, which has altered the slates for some distance into cornubianite, but without any foliated schists at all. In fact, so far as I could see, the Buller River nowhere, from its source to its mouth, runs through true foliated schists, but only through these slates and sandstones pierced by granite, or through still younger sedimentary rocks. This discovery makes a great change in our ideas of the geology of this part of New Zealand, for it destroys the supposed continuity between the foliated schists of Westland and those of Nelson. To enter into more detail: Both sides of the valley of the Lyell River are composed of sandstones and silky slates like those of Reefton, and it is in these rocks that the United Alpine Gold-mine is situated. Boulders of granite are found in the river-bed, but I did not see it in position. Neither did I see the large block of Cretaceo-tertiary rocks shown in the Geological Survey map (1886) on the left bank of the river. A little below the bridge on the Nelson and Reefton Road a well-marked dyke of granite, 2ft. to 3½ft. thick, crosses the Lyell. It can be seen in the cuttings on both sides of the river, traversing slates and sandstones. At the south-eastern end of the town there is a large mass of granite, and between here and the first creek on the Nelson Road—a distance of about a mile and a half—seven bands of altered slate alternate with granite (see cut), the third slate-band from Lyell containing Section on the road east of Lyell. Distance about one and a half miles. a. Granite. b. Cornubianite. a dyke of granite 2ft. thick. Beyond the first creek we again find granite, then altered slate, and again granite

crossed by a dyke of hornblende dolerite. About three miles from Lyell we come upon unaltered Maitai slates, followed at five miles by pink and white granites. When a junction between the granite and slates can be well seen it is always sharply defined, and the granite is generally altered for from 1in. to 6in. in depth into a finergrained rock, with little visible mica. The altered slate is highly micaceous, and is sometimes reddened for a foot or so from the granite. The evidence is therefore conclusive that the granite is eruptive. The granite is a rather coarse-grained rock, with white felspar and both muscovite and biotite in about equal quantities, and abundant. Its specific gravity is about 2.64. Under the microscope, with polarized light, the quartz shows as a rather fine granular mosaic (“granulitic,” of Michel-Lévy), with larger pieces scattered about. Gas- and liquid-cavities are present as usual, and the larger pieces have delicate pale-green hairs of an undetermined mineral running through them rather abundantly, as well as small prisms of apatite. The felspar appears to be all orthoclase, often showing cross-hatching due to the presence of microcline. The muscovite and biotite show their usual characters in convergent polarized light. Iron-oxides are very scarce. The granite boulders in Lyell River differ from that in the road-cutting in containing plagioclase and in having less muscovite, but the quartz is the same in both. The altered granite, near the contact with slate, is finegrained and brownish-white, with the mica not conspicuous. Specific gravity, 2.67 to 2.78. Polarized light shows the quartz in much finer grains than in the unaltered granite, but it still exhibits the peculiarity of two sizes. Muscovite is present in small crystals, and is rather abundant, but biotite is absent or rare. Felspar is not recognizable as crystals, but forms a kind of ground-mass of a greyish colour, with bright points of quartz scattered through it, thus approaching an elvanite. Higher up the Buller the main mass of granite agrees with the boulders in the Lyell River, so that it would seem that the main mass is a plagioclase - biotite granite, but that towards the western margin the potash minerals, muscovite and orthoclase, are developed at the expense of the soda-lime and magnesia minerals, plagioclase and biotite. The hornblende dolerite is a dyke in the granite about two miles from Lyell on the Nelson Road. It is very tough and difficult to break so as to get unweathered specimens; compact, black, showing a ground-mass with irregular crystals of augite and altered olivine. Specific gravity, 3.04. The microscope shows an almost holocrystalline ground-mass, sometimes

clouded with chloritic granules. This ground-mass is composed chiefly of felspar laths, augite, and small crystals of brown hornblende, with some magnetite. In it are a number of colourless needles, probably of tremolite, often arranged in bundles, and which penetrate the felspar laths. The hornblende is in six-sided prisms, strongly pleochroic, and showing prismatic cleavage. It is mixed with small crystals of pink augite and the other materials of the ground-mass. The porphyritic minerals are augite and olivine. The augite is in large pale-pink crystals, which are nearly colourless in the interior, but with a broad pink external zone. A very good section parallel with the clino - pinacoid gave the angle c: γ at 38°. The olivine is also idiomorphic, and occasionally quite fresh, and then pinkish-olive in colour; but usually it is completely changed into serpentine, although easily recognized by its characteristic method of decomposition. The slates of the district are of the ordinary type, common on the west coast of New Zealand, and require no special description. They are more or less arenaceous, and some-times slightly fissile, but never show true cleavage; consequently they would be called mudstones by some geologists and argillite by others. The specific gravity is about 2.66. In colour they are slaty-blue or greenish. I saw no red or purple slates. The cornubianites are very interesting and are well displayed near Lyell. I recognize two kinds: the second is either the same as the first but less altered, or perhaps it was originally a more arenaceous variety:— (1.) A fine-grained, dark-grey, tough rock, sparkling on a fresh fracture, and showing mica and quartz. A subfoliation is sometimes developed, but more often it is absent. It never resembles a mica-schist. The specific gravity is 2.76. A thin slice shows quartz in small grains, but occasionally in larger pieces, and then apparently filling up vacuities. Both biotite and muscovite are plentiful; usually the biotite is the more abundant. There is no felspar. (2.) Fine-grained, greenish-grey, sparkling on fracture, but the mineral components not so conspicuous as in the last. Thin slices show abundant fine quartz-grains, with scattered crystals of muscovite, but no biotite nor felspar. Its specific gravity is 2.71.