84 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 18, 



to obtain a clearer and more definite insight even into one of the 

 lesser operations of nature. I will therefore first describe the 

 roads briefly, and only so far as is necessary for my purpose, doing 

 so, moreover, almost entirely in the words of Maccnlloch, Darwin, 

 Lyell, &c. ; secondly, I will give the explanations of earlier writers ; 

 and lastly I will point out what I believe to have been the true 

 modus ojperandi. 



The parallel roads or shelves are three in number on each side 

 of the valley; the corresponding ones on the opposite sides are 

 exactly at the same level; and all are perfectly horizontal. So 

 regular, indeed, are they that they irresistibly remind one of lines 

 ruled in a copybook, however incongruous and farfetched such an 

 idea may be. 



The sides of Glen Eoy are steep and have an equable slope. " The 

 natural 'rock," says Macculloch, "is bnt rarely seen"*; and Mr. 

 Darwin observes that " the shelves entirely disappear where cross- 

 ing any part of the mountaius in which the base-rock is exposed" f. 

 The loose materials of which the slope is formed " have," in the 

 words of Macculloch, '•' evidently descended from the hill above." 

 That this is their origin, and that they are not transported materials, 

 is plain, since they are not rounded, and since they exactly resemble 

 the natural rock, which is of a remarkable character, consisting of 

 mica slate traversed by numerous veins of red granite — a rock which 

 is limited to the upper part of the glen, and is not found in the 

 neighbouring hills J. 



" The parallel roads, shelves, or lines, as they have been indiffer- 

 ently called, are most plainly developed in Grlen Roy. They extend 

 in lines, absolutely horizontal, along the steep grassy sides of the 

 mountains, which are covered with a mantle, unusually thick, of 

 shghtly argillaceous alluvium. They consist of narrow terraces, 

 which, however, are never quite flat, hke artificial ones, but gently 

 slope towards •- the valley, with an average breadth of about sixty 

 feet"§. 



" In general," says Macculloch, " sixty feet may be assumed as 

 an average breadth ; by far the greatest portion of aU the lines will 



be found to conform to this measurement" || "The 



extreme breadth 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 " i[. " On 

 the slope of a brown hill in Glen Pintec " (he says elsewhere)** 

 " they are particularly worthy of remark on account of their con- 

 tinuity, preservation, and the almost absolute equality of their 

 dimensions, not only through the course of each individual line, but 

 respectively to each other." This uniformity of width is a remark- 

 able feature of the shelves, which has struck most observers, but of 



* Geol. Trans., Ser. 1, vol. iv. p. 320. t Phil. Trans. 1839, p. 40. 



X MaccuUoch, I. c. p. 320. § Darwin, I. c. p. 39. 



II Loc. cit. p. 322. m. i. c. p. 337. 

 ** L. c. p. 328. ^ 



