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Rh ART. XVII. Contact Rocks from West Nelson.

By Dr. P. MARSHALL, JI.A., D.Sc., F.G.S., University of Otago.

[Head before the Omyo Institute, Toth November, 1908.

DURING a visit to Collingwood and West Wanganui, Mr. E. D. Isaacson recently collected certain rock-specimens which contain minerals formed by contact metamorphism, hitherto not recorded from New Zealand. All geologists who have visited this district have recognised the fact that large granite intrusions have been forced through older (probably Paleozoic) sediments. The granitic masses of part of the district have lately been accurately mapped by Dr. Bell and his assistants. Messrs. Webb and Clarke. During the course of this work certain altered sedimentaries were encountered near the zone of contact, and in some of them andalusite was recognised.

The specimens how referred to are as follows:-

1. CHIASTOLITE SLATE. Cleavage is highly imperfect. The chiastolite is not recognisable in hand- specimens. There are irregular grey spots } in. in diameter, which from their softness appear to consist of some secondary mineral. The section shows that the greater part of the rock consists of minute grains of quartz mingled with graphite-specks. The chiastolite crystals are as much as 0-15 min. in diameter, and twice as much in length. They yre perfectly fresh, and contain the characteristic inclusions of carbonaceous matter with a cruciform arrangement. Small crystals of staurolite are rather abundant, with the characteristio pleochroiam and birefringence. The section contains much pyrite. The areas which appear grey in the hand-specimens become a pale-brown in section, with a high birefringence. They appear to be pinite psendomorphs after cordierite. Other specimens of chiastolite slate are abundant as boulders in the gravels of the forere River, but sections show that the large eristals of chiastolite 100 min. in length and 10 mm, broad have been completely changed to crowded plates of muscovite.

2. CORDIERITE SCHIST.

Hand-specimen black, with distinct fractured black erystals. Section: The greater part of the rock consists of cordierite, perfectly clear and fresh, but crowled with minute inclusions of biotite and prrite Fluid-cavities are also common. None of the inclusions are surrounded with plcochroie halos. Some of the cordierite crystals are trillings, as shown in the figure. The crystals are 60 mm. long and 30 nun, wide. The rest of the rock consists chiefly of a reddish-brown biotite ip irregular plates, Regular plates of muscovite are fairly common, and there is much pyrite. There is very little quartz.