some cases over cavities only an eighth of an inch across, it is forced down into them, and strikes the little cliffs or slopes which are opposed to the direction of its motion at a lower level than it left those on the other side,—which latter ones remain sharp and unrounded. This can frequently be observed, even in most minute fractures, upon glaciated rocks which the ice has not long quitted.[1] Fig. 5, p. 144, represents an example; the arrow points out the direction in which the glacier has moved, B the weather, and A the lee side.
This affords a means of distinguishing glacier from water action in hand specimens of rock.[2]
2. There is reason to believe that if glaciers were to move with rapidity, instead of with such extreme deliberation, angular surfaces would not be rounded, but flat surfaces would be produced from the beginning. That is to say, instead of turning out surfaces, such as are shown in the section, Fig. 3, p. 144, after many centuries of work, glaciers might produce similar ones to Fig. 4, or even flatter, in the course of a few hours. The amount of flatness which would be produced would depend upon the rate of the motion and the bulk of the ice.
Professor Steenstrup, of Copenhagen, read to me in 1867, from an unpublished MS. in his possession, a highly interesting account of some extraordinary effects which were produced in Iceland, in the year 1721, by glacier in rapid motion. It seems that in the neighbourhood of the mountain Kötlugja, in the extreme south of the island, large bodies of water formed underneath, or within, the glaciers (either on account of the interior heat of the earth, or from other causes), and at length acquired irresistible power, tore the glaciers from their moorings on the land, and swept or floated them over every obstacle into the sea. Prodigious masses of ice were thus borne for a distance of about ten miles over land in the space
