THE PARALLEL ROADS OF GLEN ROT. 
385 
What the Macgillicuddy Beeks did in Ireland, Ben Nevis 
and the adjacent mountains did, and continue to do, in Scot- 
land. We had an example of this on the morning we quitted 
Boy Bridge. From the bridge westward rain fell copiously, and 
the roads were wet ; but the precipitation ceased near Loch 
Laggan, whence eastward the roads were dry. Measured by the 
gauge, the rainfall at Fort William is 86 inches, while at 
Laggan it is only 46 inches annually. The difference between 
west and east is forcibly brought out by observations at the two 
ends of the Caledonian Canal. Fort William at the south- 
western end has, as just stated, 86 inches, while Culloden, at its 
north-east end, has only 24. To the researches of that able and 
accomplished meteorologist, Mr. Buchan, we are indebted for 
these and other data of the most interesting and valuable kind. 
Adhering to the facts now presented to us, it is not difficult 
to restore in idea the process by which the glaciers of Lochaber 
were produced and the glens dammed by ice. When the cold 
of the glacial epoch began to invade the Scottish hills, the sun 
at the same time acting with sufficient power upon the tropical 
ocean, the vapours raised and drifted on to these northern 
mountains were more and more converted into snow. This slid 
down the slopes, and from every valley, strath, and corry south 
of Glen Spean, glaciers were poured into that glen. The two 
great factors here brought into play are the nutrition of the 
glaciers by the frozen material above, and their consumption in 
the milder air below. For a period supply exceeded con- 
sumption, and the ice extended, filling, Glen Spean to an ever- 
increasing height, and abutting against the mountains to the 
north of that glen. But why, it may be asked, should the valleys 
south of Glen Spean be receptacles of ice at a time when those 
north of it were receptacles of water? The answer is to be 
found in the position and the greater elevation of the mountains 
south of Glen Spean. They first received the loads of moisture 
carried by the Atlantic winds, and not until they had been in 
part dried, and warmed by the liberation of their latent heat, 
did these winds touch the hills north of the Glen. 
An instructive observation bearing upon this point is here to 
be noted. Had our visit been in the winter we should have found 
all the mountains covered ; had it been in the summer we should 
have found the snow all gone. But happily it was at a season 
when the aspect of the mountains north and south of Glen 
Spean exhibited their relative powers as snow-collectors. Scan- 
ning the former hills from many points of view, we were hardly 
able to detect a fleck of snow, while heavy swaths and patches 
loaded the latter. Were the glacial epoch to return, the rela- 
tion indicated by this observation would cause Glen Spean to 
be filled with glaciers from the south, while the hills and 
VOL. XV. — NO. LXI. C C 
