1 64 GEOLOGY OF THE SON VALLEY, ETC. 



the present map — were cleared up. It was found that the Iime^ 

 stone was of transition age and formed a line of low hills diverging 

 at a small angle from a similar line of hills formed by the basal 

 conglomerate of the lower Vindhyans. South of Kosai these two 

 lines of hills ccme together and there is one of which the western 

 half is composed partly of limestone and partly of the sandstone of 

 the lower Vindhyan basement beds. The eastern extremity of this 

 hill is composed entirely of the sandstone and no limestone is seen 

 to the east, its disappearance being due to its removal by denudation, 

 previous to the deposition of the lower Vindhyans which overstep 

 the eroded edges of the transition strata. 



After crossing the Rewah boundary into the Mirzapur district 



Basal Hmestone in outcrops of a limestone band at the top of the 

 Mirzapur. basal stage begin to appear. This can be traced 



continuously to the exposure of limestone in the Son at Ghurdah* 

 In the old survey this limestone was represented as No. 7 of the 

 sequence established by Mr. Mallet further east, the later survey 

 has established its true horizon and explained the origin of the 

 mistake. 



About Bargawan these transitions near their northern boundary 

 contain a number of hard bands which, in the field, resemble those of 

 the porcellanite stage and were taken for them. The limestone, being 

 newer than these hard rocks, was naturally ascribed to the horizon 

 No. 7. The more detailed survey lately made with the help of the 

 accurate maps now available, showed that the true boundary of the 

 lower Vindhyans lay further north than it had been drawn on the 

 very imperfect maps used at the time of the original survey. A 

 microscopical examination of the porcellanite beds in the transitions 

 shows that they are of a cherty nature and show no indications of a 

 volcanic origin. 



At Ghurdah the lower Vindhyan outcrop is again' reduced in 



width and no sign of the porcellanites is to be seen. Though no fault 



could be traced on this section, it is probable that these beds are here 



cut out by a strike fault, for they are well developed a short distance 



( 164 ) 



1 



