$2 LA TOUCHE : GEOLOGY OF WESTERN RAJPUTAftX 



sanidine. Very often, however, .a more or less distinct striated twin- 

 ning is developed in them somewhat similar to the twin structure m 

 microcline, but without its reticulated appearance. The striae some- 

 times lie parallel to the direction of schillerisation of the felspars, 

 and it is possible that their development is of secondary origin and is 

 connected in some way with the decomposition of the mineral. In one 

 ease (No. n'556) some of the felspars consist of undoubted oligoclase. 



Very commonly the whole of the felspar phenocrysts have been 

 converted into white powdery kaolin, which has hardly any action on 

 polarised light. In- other cases they are much clouded by dusty matter 

 and lines of inclusions, the products of schillerisation. The inclusions 

 are sometimes arranged in zones parallel to the edges of the crystal. 



Phenocrysts of hornblende occur in a few specimens, principally 

 from the range south of Jasol. They are generally in the form of 

 irregular patches consisting of aggregates of minute microlites, but 

 some show well defined crystalline outlines. They are generally 

 associated with much opaque magnetite, and are frequently altered 

 into a light yellow chloritic mineral. 



The slide cut from specimen No. T1564, from the range south of 

 Jasol, contains a well defined crystal giving brilliant colours in polariserJ 

 light, and with a high angle of extinction, which may be augite. 



At first sight in many cases the groundmass of these lavas appears 

 to be entirely glassy and homogeneous, but the application of polarised 

 light shows that It is always crystalline, though the size of the crystal 

 varies greatly, from the most minute grains, such as can be detected 

 only with a very high power, to an almost granitic holocrystalline 

 structure. In the most common varieties the quartz appears to have 

 acted as a matrix, in which minute crystals of felspar are imbedded, 

 sometimes irregularly, giving rise to a structure similar on a small 

 scale to the ophitic structure of some basic rocks ; in other cases more 

 or less parallel to each other, forming an extremely minutely grained 

 micropegmatite (PI. IX, fig. 6) ; or again arranged in radiating fibrous 

 aggregates or sphaerulites. Tie minute microlites of felspar are often 

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