Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/375

 point out the immediate sources whence they were derived. They are to be observed at the places where the larger rivers enter the glen, and it is in these points only that they possess the same elevation as the lowest of the lines. There are no remains of any terrace at a higher elevation, or at either of the two uppermost lines.

Now although the action of the present water flowing in that part of the bottom of Glen Roy which has the character of a strath, does actually produce the abrupt forms of the present terraces, it does not follow that this cause has acted throughout the whole valley. It would require a stretch of imagination beyond our power, to suppose that a successive action of water should have produced three lines, and these only (for the exceptions are much too trifling to affect the argument) at fixed intervals, through a space so large, without leaving their intermediate marks. Had they been the result of the gradual action of water on a solid terreplein, the consequences must have been similar in the higher elevations to what they are in the lower ones at the present day. We now see at the upper end of the glen numerous terraces of different altitudes produced by a series of wasting actions which I have elsewhere explained. These are destined to form marks more or less permanent on its sides, and had such been the actions generating the upper lines, the remains of similar terraces should still be visibly intermingled with them. It is true that I have noticed some such appearances, but they admit of an easy explanation on the hypothesis next to be considered, since they will generally, if not always, be found in the vicinity of torrents which have entered the lake assumed by that hypothesis, being evidently the consequences of deltas or terraces, which like those at the present head of the glen and at the entrance of Glen Turit, have been worn down by the subsequent action of water, and are no further connected with the principal lines than the present terraces that occupy the bottom.