Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/381

 admit of explanation at present; but they do not admit it on any view of the subject, and it is not essential to the truth of any hypothesis that it should explain every fact, unless we were in possession of all the collateral and posterior circumstances which may have modified or altered the actions of the presumed cause. A few of them indeed do admit of explanation, and will be considered when the causes of the present terraces are investigated.

It is necessary to describe the manner in which the water of a lake is supposed to act in producing its shores, or in which the assumed waters must have generated the lines of Glen Roy. This action consists first in checking the constant and gradual descent of the alluvia of the hills. The descending matters thus losing a large portion of their weight by immersion in the water, and in winter often rendered still more buoyant by being entangled in ice, are thrown back against the face of the hill by the incessant action of the superficial waves, and are thus evenly spread against its side, producing an inclined shore, proportioned in breadth and declivity to the various circumstances already enumerated and to the length of time during which the action has been continued. This kind of levelling action is easily seen in many of the highland lakes, and is conspicuous in particular on the prevailing lee shore, wherever such a one exists. Loch Rannoch offers a striking example of it at its eastern boundary, since its situation subjects it to a very disproportionate prevalence of westerly winds, accompanied by a corresponding power in the waves which break on that margin. It is next necessary to consider in what respect the large terraces of Glen Roy can be connected with the supposed existence of a former lake occupying that valley.

Wherever a river is found entering an existing lake, it is skirted by a shore wider than the general shores of that lake. This accumulation