OF THE PARALLEL ROADS OF LOCHABER. 
699 
explanation would seem to me to involve a greater uniformity and a higher angle on 
the slopes than actually exists. 
I cannot, in fact, see that any theory of wave action will enable us to explain the 
phenomena. If the original slope a, b, of the hillside were, as in fig. 8, B (p. 696), less 
than that of the angle of repose of the detritus, the wear of it by wavelet erosion 
would, as the materials were gradually removed, give rise to a slope at m greater than 
that which existed above beyond the reach of the fall, wdiile the debris falling from m 
into the lake l, would equally, as they were carried over the shelf r, form at n a slope 
which would also be that of the angle of repose; and that being the same in water as 
in air, the slope of m would be the same as that of n, and greater than that of the 
line a, b. But with one or two local exceptions, no such indent as m exists, and 
nowhere does a ridge such as formed by n occur on any section of the parallel roads. 
If, on the other hand, the line a, b (fig. 10) had been that of the angle of repose of 
loose detritus, the upper slope a, r, would, it is true, as it crumbled back on the 
removal of the debris by wavelet action, continue unchanged through m ; while the 
a, b. Original slope, taken as that of the angle of repose of loose debris, m, n. Slopes after transfer 
of portion of debris from upper to lower part of slope. 1. Lake level. R. Shore line. 
fallen debris, precipitated over the shelf R, would roll down on the lower slope R, b, at 
the same angle of repose as it had above, so that the fresh slopes m and n would be 
themselves parallel, and also parallel to the original slope a, b, and the theoretical 
parallelism of Macculloch might in this way be obtained. But here again it is to 
be objected that the slope a, b is never that of the uniform slope of loose detritus, but a 
variable slope having angles with the horizon, ranging, as shown in Table I., from 19° 
to 36°, all of which are less than that of the angle of repose of debris of this character. 
We have in the detrital mounds of the Turret a measure, or a closely approximate 
measure, of what this angle is. There, where the mound has been worn back by the 
Roy, the loose angular and subangular fragments of the upper beds, which are com¬ 
posed of very similar materials to those covering the hill sides, have fallen down and 
form sloping banks of which the angles vary very little from 40°, which is greater 
than any of the recorded angles. 
