OF THE PARALLEL ROADS OF LOCHABER. 
669 
Not only, however, is there the difficulty of conceiving a sufficient vis a ter go as to 
force these two glaciers, after crossing their respective valleys, up the opposite heights, 
but considering the breadth and direction of these valleys it is almost impossible to 
avoid the conviction that their whole mass would have taken the lines of least resist¬ 
ance, and turning laterally into Glen Spean and the Great Glen, have joined the 
ice-streams escaping through them.* That this was the case with the Glen Treig 
glacier is I think, as presently explained (p. 684), shown by the direction of the ice 
scratches in that part of the Strath Spean immediately opposite the glen. 
Mr. Jamieson does, in fact, carry a portion of each glacier for some distance down 
these valleys, so that one blocked up the entrance of Glen Roy, and the other the 
entrance of Glen Spean—but in doing so he seems to me to attach too much 
importance to their individual action. 
The formation of the small lake of the Merjelen See, in the Swiss Alps, has been 
adduced in illustration of the power of glacier barriers. But this lake is under 1 mile 
in length, by \ mile wide, with a mean depth of 22 to 25 feet and of 114 feet next 
the ice;+ and although it is supported by the Aletsch glacier, which descends 5 miles 
lower, and has opposing buttresses in the mountain slopes on the other side of the 
glacier a mile wide, nevertheless the water escapes almost every year, and the lake 
drains in the course of a few hours through crevasses in the great glacier. Another 
season, however, repairs or removes the breach, and the lake refills. But it must be 
borne in mind that the source of the glacier is much above the level of the lake, and 
that the dimensions of the lake bear no proportion to that of the glacier, so that the 
escape of this comparatively small volume of water would have little effect on so large 
a mass of ice ; and that the lake is not in the path of the glacier. 
These Locliaber lakes, on the contrary, were respectively 5, 10, and 22 miles long, 
and at least 700, 800, and 650 feet deep at their lower end. With the hydrostatic 
pressure due to bodies of water of these heights, no single glaciers, especially when 
operating at their weakened extremities, seem competent to deal. 
Not only so, but even with glaciers filling the larger valleys, although small lateral 
lakes are often formed, as in the case of the Aletsch glacier, and as appears to be more 
common with some of the Himalayan glaciers, these lakes are never permanent for a 
length of time. It would seem that their escape is not usually effected by rupture of 
the glacier, for after the event the glacier remains apparently unaltered, but by the 
circumstance that while at one time the ice of the glacier is compact and unbroken, 
at other times it is fissured and broken, so that when in the progress of the 
glacier a continuous fissure is brought hi front of the lake, its waters at once escape 
with greater or lesser rapidity, but never with the suddenness of the bursting of a 
* [In a letter recently received, Mr. Jamieson explains Lis views on this point more fnlly. He says that 
he considered there had been an immense accumulation of ice in the valley of the Spean opposite the 
entrance to Loch Treig, and that this ice had set out from that point as from a summit level or watershed 
in two great streams, one of which, with the united ice drainage of the lateral glens, took an eastward 
route by Loch Laggan to the Spey basin, while the other flowed westward down Glen Spean.—Oct. 1879.] 
t Dollfus-Ausset, ‘ Materiaux pour l’Etude des Glaciers,’ vol. v., p. 458. 
