and Marlborough Railways. 193 



lower than it is to the westward. The passage by which the drain- 

 age' leaves the valley at Crofton Engine, is 116 feet higher than 

 the corresponding point at Rushall. It is now, after having been 

 considerably deepened by the action of the stream, a wide open 

 valley like many others at equal or greater depths in the chalk ; 

 and its formation by the same process as that suggested in the case 

 of the Avon valley offers no difficulty. 



The conclusions to which the above considerations have led me, 

 may be shortly stated thus : — 



The Yale of Pewsey was excavated to a great extent by marine 

 denudation. 



The period at which this was accomplished was between the 

 deposition of the Lower Eocene beds, and the Boulder clay ; the 

 valley being of the same age as the great Chalk escarpment. 



The drainage, turned back by an oscillation of level, found its 

 way through a gap between the valley, and the head of a coomb, 

 caused in a great measure by marine denudation. 



The time of the reversal of the drainage was probably soon after 

 the emergence of the valley from the sea, certainly before the 

 post-pliocene period. 



The lowering of the watershed near Lydeway by 90 feet at least, 

 since the river first flowed through the Avon valley, is due rather 

 to an oscillation of level than to sub-aerial degradation. 



Finally, although the conclusions to which I have arrived may 

 be disputed, the facts on which they are founded, are I believe, 

 incontestable, and are available for other attempts to trace out the 

 Geological history of the district. It is only after repeated attempts, 

 and perhaps failures, that the whole truth can be elicited. 



VOL. IX. — NO. XXVI. 



