Page:Popular Science Monthly Volume 6.djvu/695

Rh Strongly in favor of this view are the erratic bowlders and blocks of stone, and the heaps and ridges of drift, so widely scattered over the mountains, plains, and valleys of Europe and North America, precisely similar as they are to those left by the retreat of the glaciers at the present time. And then, as now happens, the melting of floating icebergs, that were detached from the foot of glaciers as they reached the coast, strewed the ocean-bed with stones, gravel, and mud. The wide plains of Northern Germany are abundant in rocks and gravel from Scandinavia and Finland, for example. Our surface-deposits, therefore, are simply the detritus and débris of mountain-regions, transported thence by glaciers, and spread over our lower levels by the rivers that the melting of these glaciers produced.

This theory was first definitely propounded by Venetz, and has since been developed and verified by Charpentier, Agassiz, Forbes, and many others. Many geologists have opposed it from the first, but it may now be regarded as of practically universal acceptance, and as gaining constant confirmation from the immense number of facts annually observed and published.

The glacial theory implies the former prevalence in Europe and North America of a climate marked by much snow and rain, as well as ice; and this is confirmed by the characteristics of the fauna and flora of the time. In addition to the waters produced by the melting of glacial masses then covering so large a portion of the Northern Hemisphere, the very great rainfall incident to such a climate would swell the volume of the great currents of the period—currents not of transient flow like mere mountain-torrents, or our local freshets, but that swept on for centuries or millenniums. Minor additional inundations, also, would result from risings and subsidences of the earth's surface in given localities, from the damming of the waters in the valleys by glaciers and avalanches, from the sudden emptying of mountain-lakes thus formed, and possibly from earthquakes. It is to these various causes that we may attribute the washing out of the lower terraces of our present river-valleys.

The great currents by which we explain the various phenomena of the drift are due to the glaciers of this Ice period, then; and this suggests the further question, What produced the Ice period itself, the long-prevailing low temperature of regions now warm or temperate? A vast array of observations commends to the attention the following answer:

There was then a distribution of land and water upon the earth very different from the present, and, as the result of it, a different system and direction to the currents of the sea and air. And there are