Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/347

 words, will render easily intelligible. There is no need for the pickaxe and spade to investigate this question, since the water courses have produced innumerable natural sections; recent ones may be found after every fall of rain, a circumstance in which Lochaber is by no means deficient.

The extreme breadth of these lines may safely be taken at seventy feet, or a little more, and their most general one lies between that and fifty. As in no instance that I have remarked do they exceed the former, so they very rarely indeed fall short of the latter dimension. The most remarkable exception to this rule has been already noticed in describing the upper part of the glen; and it may not be amiss to repeat that the lines are narrowest and least marked on the hardest and most rocky ground, where in fact they cannot, with any latitude of language, be called roads, since they are absolutely invisible to a person when standing on them. In no case is their surface level, but it lies at various angles with the horizon, from 30° and upwards to 20° and 12°. It is probably from this cause that they are in many cases invisible where we should otherwise expect to find them; their own inclination coinciding so nearly with the general slope of the ground as to render them imperceptible from the place of the spectator. Both the interior and exterior angles are very much rounded; and the surface, I need scarcely add, is marked by considerable inequalities, from the fall of stones, and the partial accumulation of plants and recent soil. In describing their relation to the side of the hill, they may be said to bear the resemblance of sections of parallel layers applied in succession to its face. In only one instance is there a slope, resembling a superior talus, and this is visible for perhaps half a mile; while