Dr. Mac Culloch on the Parallel Roads of Glen Roy. 363 



will be found level, and they assume an artificial appearance. In 

 other cases their surfaces are irregular, and often, where the river 

 encroaches near on the original hill, are gradually blended with it 

 until they are lost in the general slope ; the river forming a second 

 and more abrupt one in the covering of alluvium which time or 

 more active causes have accumulated on its face. 



This sketch is sufficient to give a general idea of the nature of the 

 hypothesis I am now about to examine. 



Since the opposite lines of Glen Roy correspond at three several 

 stages, it is apparent that the action of the water must have consisted 

 in cutting its way through an alluvial plain, first from the highest to 

 the lowest of these stages, and ultimately to the present bottom of 

 the valley, or rather to the bottoms of all the different valleys which 

 now exhibit these appearances. 



It is easy to see that no set of partial alluvia, occupying the sides 

 of hills or the entrances of lateral torrents, could have answered the 

 necessary conditions ; as in no other case than the one previously 

 supposed, could the river have occupied the requisite elevation. 

 When therefore it is said that the lines of Glen Roy are the remains 

 of water terraces, it ought also to be shown how these terraces were 

 first formed. It is evident that the circumstances here assumed are 

 the only ones capable of terminating in the present appearances. 



It is a remarkable circumstance, on any supposition, that the 

 lines should not only be so generally equal in breadth compared with 

 each other, but that they should be so equal throughout such a va- 

 riety of ground. But on the supposition that they are the remains 

 of terraces, and that these terraces are the relics of a prior terreplein 

 which has been subsequently removed by the action of water, it may 

 be considered as impossible. The variety of ground over which they 

 are extended, and the unequal action which water must have exerted 



