22 GEOLOGY OF THE SON VALLEY, ETC. 



is here some uncertainty as to the thickness of beds which should 

 be ascribed to the Rohtas stage on account of the unusual develop- 

 ment of the nodular shales, the number of the calcareous nodules, 

 and the occurrence of more or less impure calcareous bands among 

 the shales. The bottommost beds of the Rohtas stage contain 

 bands of shale with calcareous concretions, very similar to those at 

 the top of the Kheinjua stage, so that it is difficult to draw the 

 boundary here. Still, unless the limestones in the Ghagar at Mar- 

 kundi are Rohtas and not Mr. Mallet's No. 7, it is evident that the 

 thickness of the Rohtas stage must be very much less here than 

 further to the west. South of the Kaimur scarp, where it resumes 

 its general easterly course, the thickness of the Rohtas stage is not 

 less than 700 feet and it seems to preserve this or a greater thickness 

 to the east. 



These variations may be ascribed in part to original variations in 

 the thickness of the deposits, in part possibly to 



Cause of variations. ' ■ 



removal of some of the upper beds by denuda- 

 tion, prior to the deposition of the Kaimur sandstone, but are also 

 explicable in part by the fact that the base of the stage is not a well- 

 marked horizon, but is drawn where the deposits change from an 

 argillaceous to a calcareous character. This may well have taken 

 place at different times on different sections, and the base of the 

 stage as mapped at one place probably represents a higher or lower 

 horizon than it does at other places. 



Though doubtless a partial cause of the variations in thickness 

 of the Rohtas stage, the last-mentioned explanation is not sufficient 

 to account in full for the variations. These can only be satisfactorily 

 dealt with in connection with the distribution of the rocks of the 

 upper Vindhyans, and their consideration must consequently be 

 deferred. It may, however, be noted that on the whole the thickness 

 of both Kheinjua and Rohtas stages is greatest where the distance 

 between the boundary of the Kaimurs and the southern boundary of 

 the lower Vindhyans is greatest, and least where they approach most 

 closely to each other. The one striking apparent exception to this 



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