Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/392

 not however inconceivable that the causes which are now, by the accumulation of alluvium, obliterating the existing lakes, should, under some variation of ground, have heaped a barrier in the course of a valley; and generated at one period a lake which they were afterwards destined to destroy, or which, accumulating strength by confinement, while the opposed barrier was undergoing a slow waste, should suddenly break its bounds and again desert the valley which it had been previously compelled to occupy. But the difficulty of removing the other barriers, which in this case must have remained after the breaking down of that one, continue unsolved even on this supposition; and other causes, which we know not well where to seek, must be found to explain the removal of alluvia from points where they appear at present to be, on the contrary, accumulating.

This difficulty is still augmented by examining certain phenomena connected with the lines of Glen Roy, which seem to point to a distinct and still more distant alternation of the state of the land at those places where a free communication now exists between them and the sea. These will be found to involve a set of actions even more intricate than those by which the water was originally drained from this lake.

In describing Glen Roy I have noticed the appearances of lines on the sides of the small torrents which descend from the faces of the hills. They enter into these furrows to a certain depth, but are not continued throughout the whole curvature. Now if we examine the structure and position of the rocks which form the surface of the hill at any of these points, or consider the alluvium by which they are in other places covered, we shall have no hesitation in admitting, that like all similar furrows and water courses on the faces of hills, they have been formed by the action of the rivers which now occupy them, or have been scooped out by the descent