LOWER VINDHYAN SERIES. 95 



extinguishes uniformly, or with a very slight waviness which may be 

 attributed to the same reason. It is full of glass inclusions.- Besides 

 the grains of quartz and felspar, there are a great many fragments 

 similar to them in size and irregularity of outline, and consisting 

 either of micro-crystalline rhyolite, or of a glass with imbedded felspar 

 microliths, or even of obsidian which remains without any action on 

 polarised light. 



The fine groundmass consists principally of small grains of felspar 

 and quartz and fragments of devitrified glass through which are 

 scattered flakes of biotite. Occasionally grains of calcite are met with. 



Most of the other specimens examined microscopically (-g^, 

 t^j, and g^j) are essentially similar in all their characters. The two 

 specimens ■££$ and -g^ are from a much lower horizon than the 

 "porcellanic stage," as they were both gathered from bands occurring 

 interbedded with the shales of the division No. 1 of Mr. Mallet's 

 nomenclature. Porcellanites also are found similarly situated, ^y^- 

 is from the region of Kon Khas, ^ T from the Garbandh outlier. 



The lower Vindhyan sandstones are frequently, though by no 

 Secondary growth of means ^variably, transformed into highly com- 

 quartz * pact quartzites owing to a secondary growth 



of the quartz-grains, so that the rounded grains which were originally 

 more or less independent, have been united into a compact aggre- 

 gate of intergrown crystals. This tendency is particularly manifest 

 whenever the rocks contain any volcanic material even in small 

 proportion, as if the silica contained in the colloid glass were 

 particularly liable to solution and re-deposition. To this kind of 

 action, more perhaps than to devitrification, is due the extremely 

 homogeneous aspect and compact nature of the porcellanites. It 

 has just been mentioned in speaking of the trappoids that amongst 

 the angular fragments which they contain, a great many consist 

 of micro-crystalline rhyolite. It is of course very difficult to say 

 whether this micro-crystalline structure is original or the result 

 of subsequent devitrification. There is, in favour of its being partly 

 original/ the fact that perfectly allotropic fragments of obsidian are 



( 95 ) 



