Dr. Mac Culloch on the Parallel Roads of Glen Roy. 349 



difficulty of maintaining a water level throughout the whole con- 

 nection of this interrupted and distant set of lines, is such as would 

 require a knowledge of engineering and the possession of methods 

 which we can scarcely concede to times so rude ; and that the want 

 of bridges of communication where they are interrupted by torrents, 

 of which no traces can be discovered and of which the knowledge 

 could scarcely then have been in existence, must have rendered them 

 useless as roads. Nor ought we to pass over another circumstance 

 which I have noticed in describing them ; that towards the top of the 

 glen many marks are found precisely similar in dimension, level, and 

 general aspect, but running through short spaces and at levels different 

 from those of the supposed roads. These are alone sufficient to 

 point out a different cause, and when considered together with the 

 terraces which I have already shown to be continuous with one 

 of the lines, they indicate some action of water as the real cause of 

 this phenomenon. Into the different modes by which this action 

 might have produced thern I shall now proceed to enquire. 



The visible and demonstrable marks of a continuous set of water 

 levels throughout the whole of these lines, has very naturally given rise 

 to the notion that they have been the result of the abovementioned 

 cause at some distant period. The nature of this action is however by 

 no means very easy to assign, and as the several views which may be 

 taken of it are attended with consequences more or less difficult of 

 explanation, and at any rate of very extraordinary importance in a 

 geological view, it is necessary to examine into the various ways in 

 which this agent might have produced these effects, instead of re- 

 maining content with a vague and general idea of their having ori- 

 ginated in such a cause. Only three modes of explaining the action 

 of water in producing these lines occur, and they are all derived 



Vol. iv. 2 y 



