236 pEocEEDiNGs OF a?HE aEOiOGiCAL SOCIETY. [May 12_, 



portant in reference to the glacial form of the lake theory, that even 

 in the end of Angnst the snow was still lying unmelted in the corries 

 to the sonth ; hence any climatal conditions that would produce 

 glaciers on Ben Nevis capable of damming up the valleys of the Roy 

 and Spean, would have also sufficed to fill this lofty valley with 

 an ice harrier. If ice prevented the water flowing west through the 

 Bpean, it ought also to have prevented its flowing east into the Spey. 



The next point of assumed overflow, when the lake stood on the 

 level of the second line, is the col at the top of Glen Glaister. This 

 is very nearly on the level of the second line, as is easily seen even 

 v/ithout instruments. The summit-level is a flat and marshy plain. 

 A small stream that comes down to it from the hill on the south- 

 west, after winding through it in almost stagnant pools, at length 

 flows off to the Spean by the Eough Glen. The declivity here is 

 very considerable, and the consequent rapidity and cutting-power 

 of a river flowing down it must have been very great ; yet no trace 

 of such a river appears. There is only the narrow channel of the 

 present small rivulet. A little below the watershed it crosses a 

 ridge of low rocks ; but even there no indication of a larger stream 

 is visible. According to the lake theory, the Eoy once ran down 

 this glen, as it now runs in the bottom of its own glen. We have 

 only to compare the deep, well-defined notch or gorge which the 

 river has cut for itself from the mouth of Glen Glaister to the 

 Spean with the unbroken outline of its so-called old course, to be 

 convinced that no river has ever passed through the Glen-Glaister 

 col. It is very remarkable that though there is no evidence of a 

 former river, there is evidence of a shore -line on the level, not of 

 the second, but of the higher line. A well-marked beach of washed 

 stones can be traced along the side of the hill on the east quite 

 through the pass. This shows that no barrier, damming up the 

 water to the level of the higher line, existed in this place at the 

 time when that line was forming. 



It is evident from the distinctness of the lines that the fall of the 

 water from the level of one to the level of the next lower, has been 

 on the whole sudden. On the lake theory, the fall from the upper, 

 Glen-Roy line to the second line was caused by the breaking down 

 of the barrier of detritus or ice shutting up the Glen-Glaister col. 

 Eut by removing that barrier, a depth of water of from 80 to 100 feet 

 would be set free over the whole surface of the Glen-Eoy lake, with 

 an extent of ten miles in length by above one mile in width. All 

 this enormous mass of water would be emptied out in a few hours, 

 or, at most, days. With what rapidity and what results such a mass 

 of water would escape may be imagined by any one who has read 

 the accounts of the bursting of the far smaller ice-lake in the valley 

 of the Dranse. If we wish for instances nearer home, the accounts 

 of the bursting of the Eilberry reservoir in 1852, or of that of Dale 

 Dike, near Sheffield, in 1864, will show the enormous devastation 

 and erosion a far inferior mass of water suddenly let loose can occa- 

 sion. A few sentences must be borrowed from the engineer's report 

 on the latter; ~-^' Everything solid which stood in the direct course 



