QUARTZ-PORPHYRY. to- 



In the field the quartz-porphyry is occasionally seen to be dis- 

 . tinctly intrusive, ramifying into the Delhi 



Intrusive relation to (\-.~—i^:j.„ • n . 



the Delhi guart7.it... Quartzite, as is well seen m the hilly mass 



N.E. of Jamla, also between Vasai and Malasa 

 wh&ce irregular intrusive contacts (but without extra metamorphism) 

 against the quartzite are common. This is often made apparent 

 by the porphyry directly truncating the strike of the quartzite, 

 as at the south-eastern corner of the little lake near Malasa, and by 

 inclusions of quartzite in the porphyry. The little conical boss, 

 marked 859 ft., rising about 70 feet above the plain one mile to the west 

 of Malasa (specimen No. 5 2 / ) and which is very flinty and compact 

 to the eye, contains included fragments of quartzite well seen on 

 the weathered surface (r, 2 /^). A few bi-pyramidal quartz crystals 

 are seen in the rock, but generally the quartz is completely rounded. 

 It seems likely that this conical neck is very nearly natural as re- 

 gards its outline, judging by the large number of fragments left 

 sticking to its present surface. 



The quartz-porphyry is never found in bedded or banded flows 



No bedded flows. ™ this area ' sIlowi »S that »t has never func- 



tioned as an extrusive rhyolitic lava. 

 In thin sections of the varieties of the quartz-porphyry there 



occur the usual phenocrysts of quartz, felspar 

 Microscopical appear- /,„:„,,„„}■,„ ^ j-T, i i i • » * 



anecs. (microclme, orthoclase and plagioclase) as 



well as biotite in the form of ragged strings tufts 



and long trains of aggregated flakes (12188, PI. 15, fig. 1). The "relative 



abundance of these phenocrysts varies, but they are never absent 



altogether (as in the Malani rhyolites), whilst they are generally present 



in some profusion. The quartz and felspar show good crystai outlines 



which, as is usual in such rocks, are here and there corroded by 



the solvent action of the base. Sometimes, however, the outlines 



are irregular as if the crystals had been fractured. It is noteworthy 



that microcline, which is so often the dominant felspar as a porphyritic 



clement in the granites, is also abundant as phenocrysts in" the 



porphyries. Hornblende (with blue-green and yellow pleochroism) 



can be seen among the tufts of biotite in the rock of the Likhi 



exposure 4 2 5 5 7 , 12490. PI. 15, fig. 2). 



Xenoliths, such as 4 2 ^ (12491) from near Sabli. are as common 



*, noiiths. as ' or moro s0 than ' iu the granites and micro- 



granites. The basic varieties of these xeno- 

 liths consist largely of hornblende and biotite, and in some cases 



K2 



