324 McMAHON: PETROLOGICAL NOTES ON SOME PERIDOTITES, ETC., 



No. 8 — 2S0, determined with the aid of a chemical balance, was 2*go. 

 Very possibly the porphyritic diorite of Ladakh may be the hypogene 

 representative of some such volcanic rock as the Bhandal por- 

 phyrite and its hornblende may be paramorphic after augite. 



9^—29. Hornblende-roch, from Ladakh; collected by R. Lydekker, 

 F.R.S., F.G.S. ; Sp. G. 3-095. 



This is entered in the list as a " Pyroxenite almost completely 

 changed to hornblende-rock." This short description is doubtless 

 correct and may be based on observations in the field. The rock is 

 said to be of pre-silurian age. 



Under the miscroscope the thin slices are seen to be composed 

 of hornblende with some magnetite. The hornblende is in shape- 

 less crystals or grains. In transmitted light and in thin slices it 

 varies from colourless to a pale sage-green and pale blue-green. 

 A single cleavage is sometimes well developed, but it is generally 

 very imperfect. This is crossed by transverse cracks which are 

 inconstant in direction. The angle at which these cracks cross the 

 first cleavage sometimes suggests the cross-cleavage of augite 

 and sometimes that of hornblende. Pleochroism is not strong and 

 the changes are from pale yellowish-brown to a pale brown-green. 

 The angle of extinction varies from 15 to 39 . Between crossed 

 Nicols some sections are very dull, but some polarize rather brilliantly 

 in the blue, yellow and red of Newton's first order. 



The hornblende is not at all fresh, and alteration to zoisite has 

 been more or less set up. 



There are also here and there nests of an almost colourless mica. 

 It is evidently a secondary product. 



Van Hise 1 has shown that both augite and amphibole change into 

 zoisite. The alteration of amphibole into epidote has long been 

 known and petrologists are familiar with the fact that epidote and 



1 Principles of N. American Pre-Cambrian Geology, by C. R. Van Hise, p. 690. 

 ( 22 ) 



