LOWER VINDHYAN SERIES. 93 



Chapter V.— THE VOLCANIC ROCKS OF THE LOWER 



VINDHYAN SERIES. (E. Vredenburg.) 



Various opinions have been expressed concerning the real nature 



" Porceiianites " and of the porcellanic and "trappoid" beds that 



'trappoi s . constitute a considerable proportion of the 



lower Vindhyans. Some analyses published by Mr. Mallet (Memoirs, 



Geological Survey of India, Vol. VII, page 128) show them 



to be very similar in their composition to igneous rocks, the 



coarser-grained varieties or tl trappoids " especially so. Yet, 



owing to their undoubtedly sedimentary character and the absence 



of any foci of volcanic activity, the conclusion was arrived at that 



they are results of purely aqueous sedimentation and derived from 



the disintegration of felspathic gneisses and granites, the coarser 



rocks being of the nature of felspathic sand- 

 Previous opinions. # 



stone, or arkose, the porceiianites on the other 



hand representing indurated beds of finer-divided material. Yet 

 even in the Memoir quoted above, some doubt is expressed as to 

 this being the true explanation. u It is perhaps doubtful whether the 

 highly igneous looking rock of Hoorkahoorkee may not have a 

 different origin" (page 128). The glass-like clearness of the fel- 

 spars in many of the specimens is difficult to reconcile with the idea 

 that it was derived from a granitic or gneissose rock. 



At the time of the earlier descriptions of these rocks, the micros- 

 cope had not yet been applied to their study. The examination of a 

 _, , c L , number of microscopical sections has resulted 



Rocks are of the nature r 



of the tuffs. } n showing that the rocks are tuffs. In any 



accumulation of volcanic rocks, whether acid or basic, the tuffs are 

 often found to surpass the real lavas in bulk. With acid lavas it is 

 an almost constant rule, for the eruptions are principally explosive 

 in character, and the imperfectly fluid lavas seldom flow to any great 

 distance. 



As a typical specimen of the coarser rocks or " trappoids" we may 

 take the rocks ^ T and t V°h collected by Mr. Oldham at Baheria and 



( 93 ) 



