Page:A Treatise on Geology, volume 2.djvu/29

 CHAP. VI. concavity toward the same point. "These alternate bands have all the appearance of being due to the formation of fissures in the aerated ice or consolidated nevé, which fissures, having been filled with water drained from the glacier and frozen during winter, have produced the compact blue bands."

The farther down the glacier we pass the more numerous are the fissures, the more confused the masses of ice which they separate. This arises from the inequality of the bed and sides of the channel; for thus lines of tension are produced, and across these lines of fracture. Very great fissures appear indeed in all parts of the glacier, but the displacements which these occasion as the masses move onward grow more and more remarkable, because of the additional effect of waste on the surface, in the fissures, and below the glacier.

The glacier thus slowly gliding or flowing down its channel is like a huge grinding and polishing mass. Not that the ice of which it consists can wear much even of the limestone and still less of the gneissic bed of an Alpine or Scandinavian valley, any more than pitch can wear hard speculum metal; but the glacier has under it hard stones, which, set as it were in the ice, become as effective agents in wearing away the rock as emery set in the pitch grinds the hardest compound of copper and tin. Nor is it necessary for attrition that the stones should be imbedded; their grinding effect when loose is considerable.

The sides, also, of the glacial valley are worn by similar pressure and similar agency. This is actually seen to be the case at the "angle" on the Mer de glace (Forbes), and in other situations. In fact owing to the circumstance that the glaciers in some seasons extend themselves far beyond their usual flow, and in other seasons retreat within their ancient limits, the scratched, grooved, and rounded rocks which they once covered, and between which they formerly flowed, are visible in many places, and leave no doubt of the power with which glaciers grind their channels.