Page:Popular Science Monthly Volume 30.djvu/19

Rh precipitation on the land. Hence we seem driven to the acceptance of the other alternative—diminished evaporation—for the filling of the reservoirs of the Great Basin. And when we search for a cause of diminished evaporation only one presents itself, but that offers an easy and natural solution of the problem, A depression of temperature would certainly reduce evaporation (since the power of air to absorb moisture varies directly with the temperature), and at the same time form lakes in the valleys, and glaciers on the mountains. To prove this, we have only to cite the phenomena presented by summer and winter in the Western Territories. In winter the snow-fall on the highlands is heavy, and the accumulation of moisture in this form is large; the skies are cloudy, and the evaporation is small. In summer the sky is cloudless, the heat intense, evaporation and desiccation rapid. In the spring the snows melt, flood the valleys and form temporary lakes, which in midsummer dry up to playas. A climatic change which would perpetuate the conditions of winter and spring would inevitably produce glaciers and lakes, and these would be in the main synchronous; and thus all we find recorded in the past history of this region would be repeated. But to intensify and prolong the summer would not produce either lakes or glaciers.

From the facts which have been enumerated above, it will be seen that from all sides we get evidence confirmatory of the theory that a certain period in the history of this continent was marked by the spread of ice and snow over a very much larger portion of the surface than they now occupy; and that we are fully justified in designating this time as an ice or glacial period; also that this was a period during which, from some extraneous cause, the climate was made colder, and the conditions which now prevail on Alpine summits perpetually, and in winter elsewhere temporarily, were more wide-spread and continuous.

That the Ice period was cold and not warm is also proved by the presence of the remains of an arctic flora and fauna in all regions near the old glaciers; the arctic shells of the Champlain, the arctic plants in the Quaternary clays, the reindeer, the musk-ox, the woolly elephant, and woolly rhinoceros, all tell the same story.

On the preceding pages the Ice period is spoken of as a single geological epoch of the Quaternary age: and so it must be reckoned in any general division of geological time. But the evidence is conclusive that the Ice period was double; that is, there were two maxima of cold separated by a long interval in which the climate was ameliorated, and over large areas which had been for ages occupied by glaciers and snow-fields, the ice and snow were withdrawn, and the surface was covered with vegetation, again to be partially taken possession of by glaciers.

Just how far north the glaciers retreated during the interglacial warm period we do not yet know, but probably not far beyond the Great Lakes; since the vegetation which covered Southern Ohio,