706 
PROFESSOR PRESTWICH ON THE ORIGIN 
different. It is probable that b in this section represents h and c (a very variable bed) 
in fig. 12 ; and that c corresponds with the mass d in this other section, though it 
differs from it in some respects, owing probably to some variation of the detritus on 
the slopes above ; while m (boulder clay of Mr. Brown), fig. 13, is not represented in 
fig. 12, and is probably moraine matter left by the Glen Boy glacier, like the more 
exposed mass nearer Bohuntine. The line of unconformity e is a feature to be noticed. 
The interpretation I would suggest for section fig. 12 of Mr. Brown’s, is, that d 
is the hill-side detritus, which slid down as the lake-level fell; c the sediment that 
subsided from the lake waters rendered muddy by the fall of the detritus; and b the 
subaerial debris, formed of fragments which afterwards, from time to time, fell from 
the slopes above before they became covered by vegetation. This would accord with 
Mr. Brown’s discovery of siliceous freshwater diatoms in c and d, and with their 
absence in b. The species, which were determined by Professor Dickie, are— 
Pinnularia viridis .found in c and d. 
Hemantidium undulatum ... ,, c and d. 
Surirella panduriformis ... ,, d. 
Diatoma vulgare . ,, c. 
It is important to note that Mr. Brown found no marine species, which further 
tends to disprove the marine hypothesis of the origin of the “roads.” 
On this view, the section of the Glen Boy slopes at the foregoing point on the 
4th “road,” fig. 12, would be represented by the following theoretical diagram. The 
Fig. 14, 
b. Fallen subaerial debris. c. Laminated clay (settlement from the lake waters). 
d. Slided snbaqeons debris. r. Underlying rock. 
circumstance of the rock here forming a ledge is, I consider, a chance coincidence, due 
to the irregular surface of the rock—an irregularity arising from the nearly vertical 
position of strata of varying degrees of hardness, and tending to form steps or ridges 
on the weathered surface. 
Both the inner and outer angles of the “roads” are always much rounded off, as 
shown in sections 11 and 14. 
It is to be regretted that there are no sections of the upper “ roads,” but the 
annexed rough sketch, taken nearly in front and from the opposite side of the river 
