1867.] LUBBOCK PARALLEL ROADS OF GLEN ROY. 89 



ever, as we have seen, is not the case; but, on the contrary, the 

 very even breadth of the roads is one of their most remarkable 

 peculiarities. 



Much of the confusion has, I think, arisen from the notion that 

 the roads were " continuous deltas." This idea was evidently up- 

 permost in the minds of Dr. Macculloch, Mr. Darwin, and Sir C. 

 Lyell, when they spoke of the " check given to the downward descent 

 of ordinary detritus." But in the case of a delta the check is not 

 given directly to the solid matter, but to the water carrying that 

 matter. The power of running water to carry solid matter with it 

 varies with its velocity ; and when the current is stopped, the trans- 

 lation of this matter of course ceases also. Solid matter, on the 

 contrary, rolling by its own momentum down a slope, moves under 

 very different conditions, and would not be arrested by water, but 

 (unless indeed, lighter than the water) would continue to descend 

 until it came to a slope on which it could stand. 



We see that this is the case in Glen Eoy itself, and that the 

 shelves have not been formed where streams come down the slope. 

 Not only does the matter they carry down go on to the bottom of 

 the Glen, but the streams cut back the side of the hill, and the 

 shelves are wanting exactly where they ought, on Dr. MaccuUoch's 

 theory, to be most strongly developed. 



No doubt there are cases in which terraces have been throT\Ti up 

 by waves on the side of a hill. In such cases, however, it seems to 

 me that we should have a section somewhat like that shown below 

 (fig. 4). This, however, differs from the Glen-Eoy section in two 

 important particulars. In the first place, the slope of the shelves is 



Fig. 4. — Section illustrating the form of Sea-heaches. 



away from, and not towards, the water ; and, secondly, there must 

 be a flat {a) within reach of the waves, and from which they may 

 obtain the materials to throw up. Moreover, in such a case the 

 pebbles &c. forming the shelves must be more or less rounded. 



I will now proceed to state my own idea as to the manner in 

 which the roads have been formed. 



In order to understand the matter thoroughly we must go back 

 to the far-distant time when the vaUey first acquired its present 

 depth and width. It is evident that the sides were then steeper 

 and more rugged, because the solid rock, as we have already seen, 

 seldom comes to the present surface, but is hidden by a thick cover- 

 ing of what has often been called " alluvium," or rather " detritus." 



