Again, the evidence of glacial action may be discovered in the materials themselves which have been transported by ice. Many of the large erratic blocks, after having travelled immense distances, exhibit the same sharp angular appearance as if they had only just fallen down from the cliff on the mountain side. By this circumstance they are at once distinguished from blocks of stone transported by running water; for in these the angles are sure to be rounded off by friction. Sometimes, too, they are deposited not only far away from the same rock, but in regions where no rock of the same kind exists. In the case of Icebergs, they are not unfrequently carried many hundreds of miles before being dropped into the depths of the ocean, and, in the course of their long journey, borne over the lofty ridges of submarine mountain chains. Furthermore, it often happens that a Glacier shrinks backward up the valley, and sometimes even disappears altogether. When the melting of the ice at the lower extremity exactly balances its onward progress, then the Glacier seems stationary to the eye, and occupies from year to year the same position. But, when a number of hot seasons follow one another in immediate succession, the ice is melted more rapidly than the Glacier advances, and in consequence it gradually becomes shorter, and seems to the eye to recede toward the upper parts of the valley. In this case the long lines of moraines, which before had rested on the ice, are left spread out on the plains or deposited on the slopes of the mountain. Immense blocks of stone are by this means frequently set down on the summits of lofty crags, and in such like positions to which they could not be brought by any other natural agency. These Perched Blocks, as they are called, and also those long regular mounds of earth and stones abound in several of the Swiss valleys, and constitute a very striking feature of Alpine scenery.