Page:Popular Science Monthly Volume 9.djvu/566

540 As facts accumulated, however, it was noticed that the sides and bottoms of such lakes are smoothed, in many cases polished, and almost always covered with grooves and scratches; and also that in their vicinity beds of clay are usually found, intermixed with pebbles and large bowlders which, like the rocky basins, are also smoothed and frequently scratched. It was noticed, too, that the rock from which these bowlders and pebbles had been formed commonly differed from the rocks in place on the shores of the lakes. Thus, throughout New York and Ohio, huge bowlders are common, composed of crystalline rock found in place nowhere nearer than the Canadian Highlands, a hundred miles to the northward; while the peculiar native copper of Northern Michigan is sometimes found mingled with the bowlders and striated stones of the drift far southward in Ohio.

The problem now was to discover what forces in Nature could polish and scratch both rock-surfaces and detached stones, and could also transport masses of rock, tons in weight, far from their native home.

It is well known that the loose stones and pebbles along the seashore are made very smooth and round, and often polished, by the action of the waves. It might be thought from this that the pebbles found on the shores of the lakes, and imbedded in the clays, were fashioned in the same manner. On one occasion, at the Cape of Good Hope, the writer, after wandering for a time along the sloping sandy beach of Table Bay, came suddenly to a little rocky cove exposed to the full swell of the South Atlantic. As each wave broke on the steep, rocky beach and retreated, it was followed by a sharp, rattling sound that could be distinctly heard above the roar of the waves; we noticed, too, that the stones all along the shore were in motion, rolling down the beach, only to be caught up by the next white-capped wave that came in from the ocean, and again carried up the beach, and rolled and pounded against each other by the untiring waters, that were fast reducing them to sand and dust. On examining these water-worn stones, we found them all smoothed and rounded, and often beautifully polished; but in no case could we discover, even with a magnifying-glass, any that were scratched, or in any way marked similarly to the stones which we have so often examined in the clays and hard-pans that cover so great a portion of our Northern States. From this fact, and also from watching the action of the waves on many other coasts, we conclude that the sea tends to smooth and wear away the stones and rocks along its shores, but has no power to cover them with grooves and scratches; and that, instead of wearing the coast into pockets and basins, it tends only to grind down the islands and continents to one uniform level.

Again, we have traversed the deep, picturesque valleys of the southern Alps, where we could see the glaciers glittering on the mountain-sides far up at the head of the valley, and have noticed as