LOWER VINDHYAN SERIES. 105 



Whether or not similar foci are so situated with respect to denuda- 

 tion of the overlying strata that they may come to be discovered on 

 further investigation, yet the identification of one igneous centre of 

 acid volcanic rocks of lower Vindhyan age is sufficient to remove 

 the only difficulty which, in the absence of any such recognised 

 feature, did still subsist in realising the true nature of the Vindhyan 

 *uffs. 



As a consequence of this identification, the gradations which have 

 been observed between the " trappoids n and 



Composite types. rr 



" porcellanites " on the one hand, and on the 

 r other hand such rocks as limestones, shales, or sandstones, receive 

 an easy explanation, for as the deposition of these tuffs did not neces- 

 sarily interrupt the purely aqueous sedimentation which was simul- 

 taneously progressing, it follows that when # the proportion of volcanic 

 material became smaller, intermediate types of rocks were formed. 

 Moreover, some of the eruptions may have taken place partly on 

 land, and denudation would at once act upon the lavas and loose 

 scoriae whose fragments would be mingled with the quartz-grains of 

 sandstones, or, according to their size, with the smaller fragments 

 constituting finer sediments. 



Porcellanites not quite so typical in character as those forming 

 the bulk of the " porcellanic stage " are frequently met with in the 

 division No. 1 of the lower Vindhyans. The reason for this may be 

 that the porcellanic stage represents the period of maximum activity 

 during which an enormous quantity of igneous fragments was al- 

 lowed to accumulate so quickly as to form sediments consisting entirely 

 or nearly so of volcanic materials. On the other hand, during the 

 more scattered eruptions of the earlier Vindhyan stages the amount 

 of ejectamenta may not have been so considerable as to generally 

 give rise to beds formed exclusively of such materials. This would 

 account for the more shaly appearance of so many of these earlier 

 types. 



The rock -g 1 ^ which resembles both the porcellanites and trap- 

 Rocks from the Gar- poids contains a large admixture of non-vol- 

 bandh outher. canic mater j a ] # It occurs amongst the complex 



( 105 ) 



