134 GEOLOGY OF THE SON VALLEY, ETC. 



Mr. Vredenburg, and as the section is valuable as indicating the true 

 nature of the boundary faults of the Gondwanas in this region, it 

 may be reproduced with advantage. 



The boundary with the Archaean gneiss was examined for a short 

 distance along the north-western part of map 476. It presents 

 some very peculiar features. It is almost rectilinear, and all these 

 stages of the series in turn abut against it. 



Neither original limitation nor ordinary faulting will account 

 satisfactorily for all the features observed ; for not only do the three 

 divisions severally abut against the boundary, but they occur in 

 such close proximity to one another that they must be considered 

 unconformable, or else they all three of them thin out along the same 

 line, which would be inexplicable. At the same time one good con- 

 tact shows the Barakar beds resting upon the gneiss. 



On the supposition that there is here no geological feature of 

 an exceptional character, we may consider which of the three pos- 

 sible suggestions, or whether any of them will account for all the 

 facts observed. These three explanations are : (1) original limit- 

 ation ; (2) an ordinary fault ; (3) unconformity. 



If the rectilinear shape of the boundary and the manner in which 

 it cuts off the several formations are to be accounted for on the 

 supposition of original limitation, this must be of the nature of a 

 cliff — of an inland cliff, moreover, since the Gondwanas are land 

 deposits. Inland cliffs, even when formed by some very regular 

 feature such as the scarp of a very evenly dipping series of beds, 

 are seldom strictly regular ; and in the case of a heterogenous and 

 complex series like the Archaean gneiss, it is difficult to realize how 

 the formation of such a rectilinear feature would be possible. Even 

 supposing it has been given rise to by some unknown agency, the 

 explanation does not account for the fact that the different stages 

 which appear perfectly conformable over enormous areas should 

 suddenly become unconformable along this boundary, and with dips 

 undisturbed within a few hundred yards from it, the rocks remain- 

 ing sometimes horizontal up to the very contact with the gneiss. 



( 134 ) 



