Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/361

 by the transportation of rubbish. Thus while it is engaged in deepening its own bed it is also changing its horizontal place, undermining its bank on one side and in succession quitting it to attack some other point; while at the same time it is flattening a larger portion of the plane on which it flows than the breadth of its own water, and reducing it to one uniform broad space. Thus a terrace with a profile is formed and left at a distance from the course of the stream; while a series of such capricious changes, acting through a considerable space of time both horizontally and vertically, fills the bottoms of the straths with terraces so numerous and complicated as almost to bewilder us in attempting their explanation. Such a cause may be conceived to have produced the lines in Glen Roy. We must imagine the glen filled to the depth of its upper level with alluvial matter, and suppose that a wasting stream has held its course through it for a space of time so long as to remove the whole matter to its present depth, leaving the lines which are now marked on its sides, together with the terraces that are to be seen at its upper end, as memorials of its destroying force. This is the second hypothesis depending on the action of water which has been offered in explanation of the appearances in Glen Roy.

The third and last method of explaining these appearances by the the same agent, is founded on the form assumed by the alluvial matter which in many cases is found at the edges of a lake, and on the probable consequences which would arise from draining it. On this view it is conceived that a lake had existed at the uppermost level of Glen Roy, for so long a period as to have accumulated on its margin that alluvium which now forms the uppermost of the lines in question, and that, by a subsequent sinking through two successive and similar periods, the two lower ones had been formed in the same manner. As the sinking of the waters must have been the consequence of the