Page:Popular Science Monthly Volume 9.djvu/568

 542 which fill rock-basins, with the rock-surface fresh from under the ice of the Mer de Glace, we shall find them wonderfully similar in their markings. The characters that are engraved upon them are the same.

Not only do we find these markings in connection with the present glaciers, but we find also the rock-basins themselves with the glaciers yet occupying their upper portions, and still at work grinding down the rocks. The best example of this kind, perhaps, in the world, is Lake Wakatipu, in New Zealand, which has a length of seventy miles, and a depth of 1,400 feet. This lake fills a true rock-basin, and bears every indication of having been excavated by the glaciers, which in the past were greatly extended, and have now retreated to the extreme upper end of the valley, while it has no connection with synclinal folds or volcanic fractures.

How can we resist the conclusion, then, that these bowlders, these beds of clay full of smoothed and striated pebbles, and these rock-basins with their sides covered with inscriptions—which we can now read with ease and accuracy if we take the records made by existing glaciers, as the Rosetta Stone—are all the work of glaciers, since the same results are produced at the present day by the action of ice, and by no other agency known?

A clearer idea of the manner in which a flowing glacier wears out a rock-basin can be gathered, perhaps, from the accompanying diagram, where the rock R R is shown, over which passes the glacier G,



which wears its bottom less at the lower end, not only for the reason that the ice is continually wasting away, and growing thinner in the lower portion, but also because the material carried down on the surface of the glacier is deposited at its extremity M, in the form of a terminal moraine, and thus protects the rock beneath from further waste. When the ice of the glacier is melted away, and the terminus retreats up the valley, the basin which it leaves behind it becomes filled with water (from M to G), and thus forms a lake, which may be a mere pool across which a school-boy can skip a stone, a great inland sea like Lake Erie or Lake Ontario, or a mirror of grandeur like Lake Geneva or Lucerne, in Switzerland, and Lake Wakatipu and Lake Wanaka, in New Zealand.