20 GEOLOGY OF THE SON VALLEY, ETC. 



most typical member of it is a thin bedded flaggy limestone, which 

 passes by insensible gradation into the uppermost beds of the 

 Kheinjua stage. In the east a well-marked band of shales, some 

 300 feet thick, is found above the principal zone of limestones and in 

 places in direct contact with the Kaimur sandstone. In the west 

 this shale band is not found, and the predominant rock throughout 

 is limestone, with a small thickness of shales above. In the central 

 portion of the area the exposures are too scanty to allow of the 

 presence of a thick band of shales being affirmed or denied. 



The lithology of the rocks composing this stage is very constant, 

 and has already been described by Mr. Mallet 1 and by Mr. Datta 

 (see Chapter XI), and need not further be referred to here ; but 

 a very remarkable set of shaly beds, occurring at the top of the 

 Rohtas stage and immediately underlying the Kaimur sandstone, 

 must be referred to. The greater part of these shales are argilla- 

 ceous, but towards the top they become siliceous, hard, and porcel- 

 lanic in appearance on the fractured surface. The most peculiar of 

 these, and the uppermost, is a pale coloured or white, hard, siliceous 

 rock splitting into laminae or layers of varying thickness, the planes 

 of separation being coated by a thin layer of red ferruginous 

 material. The rock itself breaks with a conchoidal fracture, is 

 compact, and presents an appearance on the broken surface not un- 

 like that of a Wedgewood mortar. 



These beds are seen at intervals from the western limit of the 

 map to about Long. 82 E. From here there is a long stretch where 

 the base of the Kaimur sandstone is not exposed, but near Hurma, 

 where the contact once more rises above the recent deposits, the 

 hard siliceous shale is not seen. In the Mirzapur district a similar 

 rock is seen, but interbedded in the Kaimurs. On the Silpi Ghat 

 the horizon was above the Bijaigarh shales, further east, about 

 Markundi, they were found interbedded with the lower Kaimur 

 sandstone. 



1 Mcm.,Geol. Surv. Ind., Vol. VII., pp. 41 ff. 

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