Igneous Rocks of SouJh Wesfern Victoria. 



13 



Carapook Rock. 



Total - i)9^49 „ 



Sp. gr. 2^67. 



Only 6 "2 per cent, of the finely powdered rock is soluble in 

 cold and 8 "3 6 per cent, in hot hydrochloric acid. 



The excess of soda over the potash in the sanidine crystals is 

 noteworthy. As a similar result is obtained in the analysis of 

 the rock in mass, it may be concluded that the large proportion 

 of soda present in the latter is derived, mainly at least, from 

 the sanidine, and not from any nepheline overlooked in the 

 microscopic slides. 



Nearly all the larger crystals embedded in the rock are 

 thin-tabular, and usually shew only the face co P cc (010), 

 tliougii I have also noticed faces in the zone oP (001) — co P (X) 

 (100). They vary in size, but seldom exceed 4 or 5 mm. in length. 

 The predominating clinopinacoidal faces are traversed by a 

 number of irregular cracks which ai^e sometimes curved, but 

 generally roughly parallel to the orthopinacoid. These cracks are 

 often tilled with tine dusty matter of a brownish hue. Owing to 

 the two easy cleavages respectively parallel to the base and 

 clinopinacoid, as well as to the cracks just mentioned, very fine 

 laminae sometimes separate when a slice of the rock with a 

 crystal at the extreme edge is mounted : between crossed nicols 

 such laminae extinguish nearly parallel to the most perfect 

 cleavage. One of my Carapook slides shews a large porphyritic 

 crystal twinned on the Carlsbad type, which, under polarized 

 light, is very similar in appearance to the sanidine twin 



