710 
PROFESSOR PRESTWICK OK THE ORIGIN 
In a previous paper on the same subject, M. Comoy (tome x., 1875, p. 8) gives 
(p. 34) as an almost self-evident proposition, “ Les terres disposees en talus com- 
mencent a glisser lorsque la force de cohesion qui les maintient sous une certaine 
inclinaison diminue par une cause quelconque et devient plus faible que faction de la 
pesanteur. ... La cause principale de Talteration de la force de cohesion des terres est 
la presence acciclentelle de Veau dans leur massif." The italics are mine.—J. P., July, 
1879.] 
§ 13. The “Minor Barriers” on the Cols or Passes at the Head of the Glens. 
I use this term to denote those smaller masses of ice and moraine debris which, 
lodged on the cols during the extension of the great ice-sheet, survived the first effects 
of the thaw, whether in consequence of their higher level, or of favourable gathering 
grounds and thicker ice. To those larger accumulations of ice and moraine debris 
extending across valleys and blocking up the entrances of large glens, I restrict the 
term “ main barriers.' 
There may have been a minor barrier on the col connecting Glen Gluoy with Glen 
Roy by way of Glen Turret, but the tributary glens are small, and in this case it is 
not essential. The one on the col connecting Glen Roy with Glen Spey is, on the 
contrary, necessary on our hypothesis, and can be accounted for by the number of 
large glens on either side of the col—glens which have their rise in mountains that 
attain the height of 2,889 to 3,700 feet. 
Suppose, therefore, no barrier to have existed on the Turret Pass, and one lake to 
have at first extended through Glen Gluoy and Glen Roy. As the fall of the water-level 
would then, on the bursting of the common retaining barrier, take place simultaneously 
in both glens, the “ roads” No. 1 in Glen Gluoy and No. 2 in Glen Roy would result 
from the same discharge ; for the fall of the water, although arrested in Glen Gluoy, 
on reaching the level of the Turret Pass, at the level of the Gluoy “ road,” would con¬ 
tinue in Glen Roy to the further extent of 21 feet, or to the level of the Roy and 
Spey Pass. If we may judge by Captain White’s faint upper water-lines in Glen 
Gluoy, the original level of this lake may have been 150 to 200 feet above that of these 
higher “ roads so that when the Spey Pass barrier gave way, the sudden fall of the 
lake-level resulted in the formation of these “roads” (Nos. 1 and 2), in the same manner 
as the bursting of the Glen Glaster barrier gave rise to “ road” No. 3. 
As I have already pointed out, both passes exhibit the appearance as of the passage 
of a large and rapid body of water, and there is an absence of the more restricted and 
narrower water channels which would have resulted from long-continued river action, 
such as the ordinary drainage of these glens must have given rise to. 
With regard to “road” No. 4, there are difficulties which I am unable to explain 
to my own satisfaction. The lower and latest lake, to which it is due, must at one 
time have extended throughout Glen Roy, and the whole of Strath Spean, as far as 
