LADAKH, NORTH-WESTERN HIMALAYA. 315 



rocks does it appear and at what period did the intrusion of 

 bowenite take place ? 



Under the microscope thin slices of the Shigar bowenite, when 

 viewed between crossed Nicols, exhibit scattered specks of doubly 

 refracting mineral matter on a dark back-ground very suggestive 

 of a star-spangled sky on a clear dark night. On revolving the 

 Nicols the bright points become dark and the dark ones bright. The 

 slices contain no isotropic matter except in veins. 



The doubly refracting particles are of microscopic size and are 

 without a trace of crystalline form. 



Thin slices contain small granules of magnetite, and are dotted 

 over with irregular opaque spots, white in reflected light, the exact 

 mineral character of which is uncertain. Some of the serpentine 

 occurs in strings having a transverse fibrous structure. 



The double refraction of the serpentine in the Shigar bowenite 

 is low. It possesses straight extinction. The character of the depola- 

 rization is positive, and its refraction lies between 1*560 and 1*606. 



The microscopic examination of thin slices does not afford any 

 clue as to the nature of the original minerals out of which the ser- 

 pentine was formed. All original structures have been obliterated. 

 Even " the curved feathery and sheaf-like crystals/' seen in the 

 bowenite of Afghanistan, and which I referred to olivine as their 

 parent, are absent in the Shigar rock. 



I give for comparison reproductions of photographs of thin 

 slices of the two rocks as seen under the microscope between 

 crossed Nicols: viz^ fig. 1, PI. 17, Shigar bowenite; figs. 2 and 

 3 small and large grained Afghan bowenite. 



No. 8—276, Serpentine after a iroctolite inclining towards picrite, from 

 Ptiga valley, Ladakh ; collected by R. D. Oldham, F.G.S. ; Sp. G. 2785. 



The hand-specimen is a dark greenish-grey compact rock spotted 

 with white. 



1 Nothing is known about the mode of occurrence of the Afghan bowenite. 



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