Marshall. — Contact Rocks from West Nelson. 101 



Art. XVIII. — Contact Rochs from West Nelson. 



By Dr. P. Marshall, M.A., D.Sc, F.G.S., University of Otago. 



[Bead before the Otago Institute, lOth 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 

 Palaeozoic) 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 now 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 mm. in diameter, and twice as much in length. They 

 are perfectly fresh, and contain the characteristic inclusions of carbonaceous 

 matter with a cruciform arrangement. Small crystals of staurolite are 

 rather abundant, with the characteristic pleochroism 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 pseudomorphs after cordierite. 



Other specimens of chiastolite slate are abundant as boulders in the 

 gravels of the Aorere River, but sections show that the large crystals of 

 chiastolite 100 mm. 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 crystals. 



Section : The greater part of the rock consists of cordierite, perfectly 

 clear and fresh, but crowded with minute inclusions of biotite and pyrite. 

 Fluid-cavities are also common. None of the inclusions are surrounded 

 with pleochroic halos. Some of the cordierite crystals are trillings, as shown 

 in the figure. The crystals are 60 mm. long and 30 mm. wide. The rest of 

 the rock consists chiefly of a reddish-brown biotite in irregular plates. 

 Regular plates of muscovite are fairly common, and there is much pyrite. 

 There is very little quartz. 



