73 L.A TOUCHE: GEOLOGY OF WE -.TERN RAJPUT ANA. 



CHAPTER VII.-PETROGRAPHICAL NOTES. 

 I. Malani Rhyolites. 



I have not yet been able to work out in detail with the microscope 

 the large mass of material collected during my three seasons' field 

 work in Western Rajputana, but I think that I have examined a 

 sufficient number of specimens to give a general idea of the composition 

 and structure of the rhyolites. 



That these rocks were exceedingly siliceous was noticed by 

 Mr. Blanford, the hardness of fresh specimens being often equal 

 to that of quartz, so that they are not scratched by a knife. Tie 

 siliceous character is confirmed by microscopic examination, the 

 ground mass always consisting to a great extent of free quartz. The 

 groundmass consists essentially of an intimate mixture of felspar and 

 quartz, with as a rule a good deal of magnetite in minute grains, and 

 dusty matter. Hornblende in the form of minute rods or acicular 

 crystals and grains also occurs in the groundmass of many specimens. 

 Augite perhaps occurs in one or two, but owing to its generally de- 

 composed condition, I have considerable doubt in identifying it ; and a 

 yellowish green chloritic mineral is common. I have not found mica in 

 any of the_ lavas, but magnesian mica occurs in minute flakes in the 

 matrix of one of the tuffs (No. U'500) 1 . Apatite is also sometimes 

 present in small quantity. Phenocrysts of quartz and felspar occur in 

 nearly all the lavas, but are in a few cases entirely absent. 



The forms of structure exhibited by the groundmass are very 

 various. Fluidal, perlitic and sphaerulitic structures are common, 

 those in fact that are peculiar to rocks that have flowed in a molten 

 condition at the surface. Vesicular structure is sometimes met with, 

 but is not very common, probably because such acid lavas as these 

 were in a very viscid condition when molten ; and some of the flows 



1 1 hcse numbers refer to the entries in the Rock Register of the Indian 

 Museum. 



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