Dr. Mac Cullcoh on the Parallel Roads of Glen Roy. 387 



There can be little doubt that in any supposable case the changes 

 themselves are of very high antiquity. Now, all the lines where 

 they are on similar slopes, in similar ground, or generally in the 

 same circumstances, present such a resemblance as to entitle us to 

 conclude, that had the ground been uniform throughout, the whoJe 

 of them would have been equal and similar. Yet lying at different 

 heights, and consequently subject to an equal action of the de- 

 scent of water, the most universally destroying of the wearing 

 causes, they must, had those wearing causes been active, have shown 

 very different degrees of injury. It is as inconsistent with the 

 action of such causes, as with the ordinary calculation of chances, 

 that an equality so great should have been preserved had they 

 been materially subjected to these causes of destruction. We may 

 therefore consider them as differing but little, even at this distant 

 period, from the condition in which they were left by the sub- 

 sidence of the water, or by the cessation of whichever of the sup- 

 posed possible causes we choose to assume as that which produced 

 them. The waste and destruction of hills is by no means therefore, 

 however certain it may be, an operation of great rapidity. We 

 are here furnished with a permanent criterion by which, within 

 certain wide limits indeed, we can estimate it. 



But this is not the only important fact pointed out by the same 

 phenomena. We have seen that the lines are formed in two 

 different sets of alluvia. The one of these is found at the upper 

 part of the glen, and consists of sharp fragments that have been 

 subjected to no distant transportation. They are the result of the 

 wearing process acting on the summits of the hills, as is proved by 

 their identity with the natural rock, by their freedom from foreign 

 mixture, by their angular integrity, and by the quantity of fine clay 

 that is mixed with them. This latter circumstance I would 



