STRATIGRAPHY. 25 



vertical beds of the lower Vindhyan series; in the other outliers this 

 direct evidence is not available, but the manner of their occurrence 

 shows that their deposition was posterior to the greater part of the 

 disturbance now shown by the lower Vindhyans, and to an extensive 

 denudation of these and the older rocks. 



In contrast to this, we find along the boundary of the main 

 area either direct indications of a conformity 



and in the main area. 



between the lower and upper Vindhyans or very 



doubtful indications of a slight unconformity. On the north side of 



the Vindhyan plateau Mr. Mallet's description x leaves no possibility 



for doubting the perfect conformity of the Rohtas and Kaimur stages, 



at any rate in the Durgauti valley. In the Son valley the sections 



are less clear, but west of Long. 82°E. the constant occurrence of a 



peculiar and easily recognizable band of siliceous shales, and the 



existence of sections showing a passage of these into the Kaimur 



sandstone above and the shales and limestones of the Rohtas below, 



indicate a conformity between the two stages. Further east we 



have a long stretch where no sections are seen, but at Hunna the 



junction is once more seen and from here eastwards to Markundi, 



though numerous sections have been seen, there is no trace of the 



peculiar siliceous shales seen further west. It is near Markundi that, 



as already mentioned, the beds recognized as belonging to the Rohtas 



stage are at their thinnest, and this might be held to indicate 



erosion previous to the deposition of the Kaimur sandstones. 



Against this supposition must be placed the fact that the few 



Possible re lacement sections which show an actual contact between 

 of Rohtas by sandstone. tne b ec j s f t h e t wo stages, though they show 



an abrupt change from the argillaceous and calcareous beds of the 



Rohtas to the arenaceous deposits of the lower Kaimur, show no 



signs of erosion. It must also be remembered that it is just in this 



area where the lower Kaimur sandstone attains its maximum 



thickness and it is not an impossible supposition that this may 



represent a part of the deposits which elsewhere are classed with 



' Mem., Geol. Surv. Ind., Vol. VII., p. 46. 



( 25 ) 



