26 GEOLOGY OF THE SOiN VALLEY, ETC. 



the Rohtas on account of their lithological composition. The coinci- 

 dence of a thinning. out in either direction of the lower Kaimur sand- 

 stone with a thickening of the Rohtas beds may be held to favour 

 the supposition, and in further support of it there is the presence, 

 both on the Hurma section and near Markundi, of a band of sand- 

 stone in the Rohtas beds, which closely resembles that of the 

 Kaimurs. 



Whether there is any truth in the suggestion hazarded in the last 

 paragraph or not, it may be taken that there is 



Lower Vindhyan debris. 



no direct evidence of an unconformity between 

 the Kaimur and Rohtas stages anywhere along the main boundary ; 

 but though there may be no unconformity here there must have 

 been one not far off. Along the whole of the scarp from Gurdah 

 to the eastern limit of the map there are found, in the lower Kaimur 

 sandstone, fragments of chert which appear to have been derived 

 from the Rohtas limestones. In places, as, for instance, north of 

 Susnai ( Susuneyee ) these are abundant enough to form a breccia; 

 the fragments are always angular or only slightly rounded on the 

 corners and are not found in the bottommost beds of the Kaimur, 

 but some way, even to a couple of hundred feet, above it. 



If derived from lower Vindhyan rocks, and there is no other 

 known source, these chert fragments indicate a considerable lapse 

 of time, disturbance, and denudation. They must have been indurated 

 into chert, the beds they were contained in elevated and the overly- 

 ing beds removed by denudation before the deposition of the beds 

 in which they are now found. 



The relation of the upper to the lower Vindhyans is consequently 

 , as fellows. Along the boundary of the main 



Relation of lower and ° ' , 



upper Vindhyans. area of the upper Vindhyans there is either 

 direct evidence of conformity between the Kaimur and Rohtas stages 

 or an absence of evidence of unconformity. Where the upper 

 Vindhyan boundary turns southwards in the Mirzapur district there is 

 evidence that the lower Vindhyans had been elevated and exposed 

 to denudation at the time the lower Kaimur sandstones were being 

 ( 26 ) 



