ing in the very centres of the valleys. They must have been exposed to the full force of the glaciers; nay, the ice-streams were evidently split by them, and had to flow upon either side and over them. "Assuredly," says Dr. Tyndall, "a glacier is competent to remove such barriers, and they probably have been ground down in some cases thousands of feet. But being of a more resisting material than the adjacent rock, they were not ground down to the level of that rock."[1] Examination of such masses has led me to form a very different opinion. The contours of their rocks, upon the sides opposed to the direction of the flow of the glaciers, are frequently flatter, and suggestive of a greater degree of abrasion, than the adjacent and lower rocks. They have been lowered more, not less, than their surroundings. Yet the indications are, as a rule, that these obtrusive crags have only been lowered to a trifling extent, and, most certainly, not thousands of feet. Still, let us suppose, for the sake of argument, that the adjacent rocks were actually softer, and were ground down a hundred or more feet upon each side of the hard crags, which, in consequence, became that amount above the level of their surroundings. The adjacent rocks would then, according to my opinion, have been prodigiously eroded; all their angles would have been obliterated; they would have become exceeding flat, and such forms as they would present would be characteristic of a high degree of glaciation. Yet we find that such is not the case. The rocks adjacent to the crags are frequently less flat, less abraded than the crags,[2] and, to all appearance, their surfaces have not been lowered more than a very few feet. The conclusions are inevitable in such cases that the adjacent rocks have suffered less than the obtrusive crags, and that any real or imaginary softness of rock has not assisted glacier-erosion to the extent assumed by Dr. Tyndall.
The enormous amount of excavation assumed by Dr. Tyndall is further accounted for by him upon the supposition that glaciers
