Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/386

 flowing through the old channels, would tend to cut them down to these levels ; whilst the sudden rise of the land would stop the further continuation of the process. It is also important to remark that at all these points a stream enters the main valley from the side, or from a lateral valley, and thus, by the debris it would bring down and deposit at its mouth, would tend to fill up the main valley to the level of the line. This is very markedly the case with the pass of Maccoul and that of Glen Glaister. It may also be noted that such coincidences of the lines with hollows between the hills are seen in other places. Such is that already mentioned of the coincidence of the upper line with the level of Loch Ericht and the watershed between the basins of the Spey and the Tay.

But it must be remembered also that difficulties do not affect only this theory. Perhaps the combination of the various conditions needed to shut and open the lake-barriers exactly at the right time and in the proper order are not more probable. If these lake-barriers were formed of detritus, its collection and sudden removal is no less inexplicable. Ice-barriers may seem more manageable, but are subject to no less inexorable conditions of climate and elevation. A glacier that would fill the valley of the Spean and Roy with a mass of ice 800 or 1000 feet thick, and some miles in extent, would require an extent of feeding- ground that is not easily found in this region. I have never heard of a lake of such depth shut in by ice in any part of the earth at the present time. To form an embankment for a water reservoir of one-fourth the depth and one-tenth the extent of the supposed Glen-Roy lakes, with the best materials he can select, is no mean task for an engineer of the present day. What would he think of the task were he required to build the barrier of a material of less specific gravity than the water to be shut in, of a material which that water and the ground on which it rests were constantly corroding and wasting away, so that his barrier had to be incessantly moving forward from behind to compensate for what it lost in front ? To form a permanent lake with a uniform level on such conditions has, I must confess, always appeared to me an almost impossible problem, far outweighing any difficulties that attach to the marine theory.

But all such considerations may be laid aside. The chief and fatal objection to the lake-theory is that it supposes rivers to have flowed in places where there is clear proof that no river has ever flowed,— that it assumes a great lake to have been suddenly drained by a narrow glen where it is undoubted no stream of water, larger or more rapid than the tiny rill gathered from the sides of the neighbouring mountain, has ever existed since the ocean laid down the loose soil spread over its smooth unbroken declivities. The theory not only fails to explain the phenomena, but is in direct contradiction to them, and therefore must be rejected.

Discussion.

Mr. Gwyn Jeffreys observed that no organic remains appear to have been found in these beaches, £o as to prove their marine origin.