LOWER VINDHYAN SERIES. IOJ 



all present secondary enlargement, by means of which their rounded 

 outline is completely lost in polarised light when the rock appears to 

 be composed of closely interlocking polyhedral grains. This 

 secondary growth even extends through many of the original grains 

 of rhyolite, which, in polarised light, appear as cloudy patches through 

 a continuous area of quartz, Many of these grains may have been 

 originally vitreous, and, as already suggested, it is probaby from 

 them that the silica has been largely dissolved and recrystallised. 

 The same action taking place right through the minute particles of 

 the "porcellanites " and of the groundmass of the u trappoids" 

 would account for their extreme compactness and their conchoidal 

 fracture. 



In addition to these volcanic strata, the division No. 1 contains 

 also porcellanoid rocks which are of a totally 



Jasper, chert. . 



different nature and consist of chert or jasper. 

 Such are the siliceous bands and segregations that frequently occur 

 in the limestones (specimen ^VV) anc ^ allied rocks occasionally 

 interstratified with the shales (specimens ■£-£$ and -gWr)» Even here 

 the silica may have been deposited by thermal waters connected 

 with the volcanic centres, but the structure is not that of a fragmentary 

 tuff. However much the rocks may resemble " porcellanites" in the 

 hand-specimen, the sections show nothing but a perfectly regular 

 fine-grained mass of quartz or chalcedony. 



In conclusion it may be said that the a trappoids " and il por- 

 cellanites " that form a large proportion of the 



Conclusion. r 



lower Vindhyan strata and whose true nature 

 has remained for a long time an undecided question, are rhyolitic tuffs 

 of varying coarseness. In one locality, between Japla and Kutumbeh, 

 felsites and true lavas have been found, which indicate the proximity 

 of a centre of eruption. These lavas occur at the base of the series 

 and in other regions tuffs occur at all stages of the division No. 1, 

 generally resembling those that form the bulk of the " porcellanic " 

 stage. Porcellanic beds of a few feet in thickness also occur here 

 and there above the main porcellanic horizon, throughout the Kheinjua 



( i°7 ) 



