g2 LA TOUCHE : GEOLOGY OF WESTERN RAJPUTANA. 



edges passes into an extremely fine grained selvage, often quite glassy 

 in appearance. Where it has been much decomposed, as in No. 

 1 1548, the olivines have almost entirely disappeared and the augite is 

 to a great extent replaced by hornblende. The specific gravity varies 

 from 3" 14 in No. 123 12, which contains a large amount of unaltered 

 olivine, to 272 in the most altered varieties. About half of the 

 specimens examined have a specific gravity of 300. 



The rock from the dyke or boss mentioned on p. 75 as probably intru- 

 sive in the Barmer sandstones near the village of Sarnu, on the eastern 

 edge of the Barmer desert, is very different in composition from that 

 just described and evidently belongs to quite a different period of 

 volcanic activity. Mr. Holland says: '' Macroscopically this rock 

 " (No. i3'8o3) is grey, with porphyritic crystals of sanidine and segerine 

 " augite, sometimes reaching (in the former case) about | inch across 

 " but more often 5 inch, | inch, or less. The weathered surface shows the 

 " characteristic pitted surface of these rocks by removal of the soluble 

 " felspathoids. The constituents are idiomorphic segerine augite, 

 "sanidine, sodalite (and? nosean\ sphene, apatite and melanite in a 

 "finegrained matrix of sanidine and nepheline. 



"Felspar. Sanidine in porphyritic crystals, fairly fresh and as 

 "numerous lath shaped crystals in the groundmass. Often in Carlsbad 

 " twins. Irregularly shaped inclusions of a colourless isotropic decom- 

 " posable mineral are common. 



" Pyroxene. ^Egerine augite variety in porphyritic crystals and 

 "as smaller granules. Zoned with zonal extinctions and hour-glass 

 "structure between crossed Nicols. Pleochroism strong, a green, b 

 "yellowish brown, £ greenish yellow. Extinction 31° (c: a): or 59° 

 " (c : a) ; occasionally twinned. Margins often rugged and of different 

 ' " colour. 



" Sphene in numerous well shaped colourless crystals, twinned. 



" Melanite. Rugged brown lumps, very few in number. 



" Socialite (and? noseari). Well shaped crystals ; dirty brown and 

 ( 92 ) 



