390 Transactions. — Geology. 



times 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 num- 

 ber 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 : y 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 ; conse- 

 quently they would be called nuidstones by some geologists 

 and argillite by others. The specific gravity is about 2-66. 

 In colour they are slat}'-blue or greenish. I saw no red or 

 purple slates. 



The cornubianites are very interesting and are well dis- 

 played 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 subfohation 

 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, greenisli-gre\', 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. 



