difficulty of maintaining a water level throughout the whole connection 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 them 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 above mentioned 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 remaining content with a vague and general idea of their having originated in such a cause. Only three modes of explaining the action of water in producing these lines occur, and they are all derived
