STRATIGRAPHY. 29 



of transition rocks north of the presumed run of this boundary fault, 

 which further west brings them into contact with upper Vindhyans 

 of the Rewah series, shows that the throw must have been largely 

 reduced either by a subsequent movement of elevation of the 

 northern side, or more probably by an original dying out of the 

 fault. 



In the latter case, we must look for the existing analogy to the 

 conditions of this area during the Vindhyan period not in the Hima- 

 layas, but in the hills of Baluchistan and Sind, where the mountain- 

 forming forces have acted with much less intensity, and where we 

 may be regarded as having to deal with the limit of an area of prin- 

 cipal mountain formation. In these hills there is no main boundary 

 fault separating the disturbed rocks of the hills from the undisturbed 

 deposits of the plains, and the beds which are confined to the margi- 

 nal portion of the Himalayas rise up into, and are involved in the 

 disturbance of the rocks composing, the mountains. Here we have 

 the rocks of the tertiary system thrown into folds, and in places 

 denuded before the deposition of nearly horizontal beds of the newer 

 series on the eroded edges of the upturned beds of the older series. 

 The conditions are, in fact, similar to those exhibited by the Vindhyans 

 of the Son valley, and we may conclude that the mountain range 

 which rose south of the present valley of the Son during the Vindhyan 

 period bore much the same relation to the Aravalli range of that 

 period which the existing hill ranges of the Western frontier do to 

 the Himalayas. 



The rocks of the Gondwana system exposed along the southern 

 margin of the area under description present no 



Gondwana system. . . ... . . 



noteworthy peculiarities. All the principal stages 

 are represented, the Talchirs only by two small outliers, and are of 

 the normal types described in the Manual of the Geology of India. 

 The boundary to the older rocks, however, presents peculiarities 

 which deserve special notice. 



From the Son to the Banas the boundary runs in a nearly straight 

 ... ,, line and is what would be called a faulted 



Boundary with older 



rocks « boundary by most observers. East of the Banas 



( 29 ) 



