8 



Prof. J. Prestwich. On the Origin 



[May 1, 



sequence of the detrital matter taking, as it successively fell and was 

 removed, the same angle of repose as that which the detrital slopes 

 originally had, that angle being the same in water as in air. But the 

 angle of the slopes varies from 25° to 35°, whereas, the angle of repose 

 of such loose materials as would fall down the slopes is as high as 

 38° to 40°. 



Besides these objections to Mr. Jamieson's hypothesis, which the 

 author considers valid, he points out the difficulty of conceiving that 

 the Arkaig glacier, which was fed by mountains not more than 2,000 

 or 3,240 feet high, and had to travel, not down a steep gradient, but 

 along a level lake basin 12 miles long, then to traverse Loch Lochy, 

 457 feet deep, could have ascended the hills at the entrance of Glen 

 Gluoy to a height of not less than 1,200 feet, while at the same time a 

 pass existed at the head of the glen only 500 feet high, which presented 

 a ready outlet to the west coast. Or, if prevented from taking that 

 course, why should not the Arkaig glacier, when it issued into the 

 Great Glen, have taken the line of least resistance and passed either up 

 or down the Great Glen, where the gradients are so small ? 



It is a question also whether active glaciers such as Mr. Jamieson 

 requires, could have formed permanent dams to the large bodies of 

 water pent up in Glen Gluoy and Glen Roy. Glacier lakes are 

 occasionally formed in the Alps, as in the instance of the Margelen 

 See, by glaciers when below the snow line, passing lateral glens whose 

 streams they dam back, but they are much more common in the 

 Himalayas. These lakes, however, never last many seasons. The 

 glacier is constantly on the move, and so long as it presents an 

 unbroken front to the lake, so long is the barrier efficient, bat when 

 in the progress of the glacier a fissured mass of ice comes forward, 

 the water at once escapes with greater or lesser rapidity, and cannot 

 again accumulate until the defective ice has travelled past or the 

 leak is repaired by winter frosts. 



In the known recent instances of such glacier lakes, they are so 

 small compared to the great mass of the glaciers, that the latter are 

 not affected by the escape of the lakes — the fissures only are 

 enlarged ; but with the great size of the Glen Roy and Glen Spean 

 lakes, and the enormous pressure exercised by columns of water 500 

 to 800 feet deep, would the preservation of the barriers and the 

 restoration of the lake be possible ? 



Equally difficult is it to imagine the existence of such vast glaciers 

 as those of Glen Arkaig and Glen Treig, while the opposite glens of 

 the Gluoy and Roy remained free from ice. For the hills around 

 Glen Arkaig are but little higher than those around Glen Gluoy, and 

 not so high as those around Glen Roy ; those to the eastward of the 

 latter valley are in fact nearly equal in height to the range on the 

 south of the Spean with the single exception of Ben Nevis and the 



