Dr. Mac Culloch on the Parallel Roads of Glen Roy. 365 



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 ele- 

 vation 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 in- 

 termediate 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. 

 Vol. iv 3 a 



