700 
PROFESSOR PRESTWICK ON THE ORIGIN 
Nor is tlie variable inclination of the “ reads,” shown in Table II., in accordance 
with the view of then’ being shore margins. Wherever, on the shores of lakes or seas, 
the slopes below water are comparatively uniform, the strand between water marks 
also presents a surface having a more uniform gradient than obtains in the “ roads.” 
Nor could the irregularity in the inclination of the “ roads” occur on the shore of a 
lake or a sea, combined with the regularity in then width which generally exists. 
Steep inclines of the strand would indicate steeper slopes below water, and steeper 
slopes below water would be accompanied by contracted width of shore line. 
Any erosion or shore-line hypothesis would also involve an interior structure of the 
“ roads,” showing the small successive increments made from year to year to the 
shelf r, in a manner analogous to false bedding or oblique lamination, and this has 
not been shown to exist. 
These objections seem to me to be fatal to the idea that the “ roads” are due merely 
to the long-continued action of water on a shore-line. There is, nevertheless, a sym¬ 
metry and uniformity of character about the “roads” which indicates some general 
cause, one independent of local conditions, which are subordinate to it, and of magni¬ 
tude sufficient to give a common inrpress to the whole. 
§ 12. Origin of the “Roads.” 
In the case of the 1st, 2nd, and 4th “ roads,” we do not know what the conditions 
immediately antecedent to their formation were; but in that of the 3rd we are in 
possession of these conditions, and are in a position to trace uninterruptedly the 
successive changes which took place from the finish of No. 2 “road” to the formation 
of No. 3 “road.” If we find in this sequence an explanation that will account for 
the formation of “road” No. 3, its application to the other three “roads,” which have 
clearly the same origin, will follow as a matter of course, provided the other con¬ 
ditions are concordant. We have therefore, for the present, to confine ourselves to 
“roads” Nos. 2 and 3, which restricts the inquiry to Glen Hoy. 
The height of No. 2 “ road” has been found to be on a level with that of the col 
between the head of Glen Roy and Glen Spey, and that of No. 3 with the height of 
the col between the tributary Glen Glaster and Glen Spean. Therefore, as there 
is a difference of 76 feet between the height of the tw r o cols, when the lake stood at 
the level of No. 2 “road,” and its overflow escaped by the higher Spey col, the lower 
col of Glen Glaster must have been blocked by a barrier more than 76 feet high. 
For the moment, it is immaterial to the question whether the barrier consisted of 
glacial detritus or of ice. It could be no other. Now it is well known that so long 
as a dam formed of gravel, soil, or clay stands above the level of a reservoir, it is secure; 
but if the water rises so as to trickle over the edge, then, unless the mischief is at 
once stopped, the water quickly enlarges its channel, and the velocity and power of 
the stream increase so rapidly that the dam is soon destroyed, and the water escapes 
