Page:Popular Science Monthly Volume 9.djvu/309

Rh One result of the formation of an ice-cap over the polar regions alternately in one and the other hemisphere might very well he, as claimed by Croll and admitted by Sir William Thomson, such great ebbs and flows of the ocean-waters as we find recorded in the Champlain clays, and the present depressed sea-level; but some more conclusive evidence of an ice-cap will be asked by cautious reasoners than these alternations of level: such evidence, for example, as universal glaciation over all of North America of 40° north latitude. No such evidence has as yet been adduced; but, on the contrary, observers report an absence of ice-marks in the interior of the continent northwest of the Great Lakes. This we might take to be proof that the glaciers of the Ice period were limited to the highlands comparatively near the ocean, the source of evaporation, and that the interior was so dry then and now that no glaciers could be formed there. This is, however, a subject which requires further investigation. Whatever be its cause, the uniformity and magnitude of the change of sea-level from the Tertiary emergence to the Champlain submergence, and then to the present, render it one of the most remarkable phenomena recorded in geological history, and one that with careful study will probably throw much light upon the great dynamical influences that have produced changes on the earth's surface.

III. Either simultaneously or alternately with the extremes of warmth and cold, which we find recorded in the northern, warm and cold periods prevailed in the southern hemisphere. The evidences of a Glacial period in South America are as conclusive as on our own continent; but it is difficult to conceive how barriers could, at that time, have been thrown across the great open oceans—the South Atlantic and South Pacific—in such a way as to confine the tropical currents to the central portions of these oceans.

We are, perhaps, not justified in saying that such barriers never did exist, but it will be conceded that the difficulties which oppose their erection there are much greater than in the northern hemisphere; and the hypothesis which supposes their existence in the Glacial period of the southern hemisphere is so entirely unsupported by facts, that we are compelled to regard it as mere conjecture.

In any discussion of the phenomena and causes of the Ice period we are, up to the present time, somewhat limited and embarrassed for want of a wider range of observation. The facts are not yet all in. Nearly all the detailed and careful observations made on the glacial phenomena of the northern hemisphere have been limited to the eastern half of North America and the western part of the European Continent. Here the traces left by the glaciers are really stupendous in their magnitude and extent; and we have demonstrative evidence that, during the Ice period, the glaciers and snow-fields of Greenland stretched continuously down along the Atlantic coast of North America to and below New York, and that the highlands of New England