Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/382

 portant in reference to the glacial form of the lake theory, that even in the end of August the snow was still lying unmelted in the corries to the south ; 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 without 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 Rough 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 Roy 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-Roy 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 Bilberry 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 occasion. A few sentences must be borrowed from the engineer's report on the latter; "Everything solid which stood in the direct course