Page:Popular Science Monthly Volume 30.djvu/17

Rh growth of trees and smaller plants. The fir-forests here meet the snow-banks in actual mechanical conflict, and the front ranks of trees, though of good size, are weighed down by the snow and grow prone and interlaced upon the ground. The snow-fields rise three thousand to four thousand feet above the snow-line, and there are miniature glaciers at the heads of the valleys—representatives of the great glaciers that once filled these valleys to their mouths. The precipitation remains, the snow-fall remains, but the glaciers are gone. Here we have just the conditions most favorable to the formation of glaciers according to the theory of those who regard glaciers as thermal phenomena, but no glaciers—because of the high annual temperature. With an increase of the average annual temperature, even with increased evaporation and precipitation, it is evident that no glaciers would form; but with a depression of temperature which should cause the rain-bearing winds from the Pacific to do all the year what they now do only in winter; viz., heap up snow on the highlands; and some of this snow-fall should accumulate year after year, the mountain-slopes and draining valleys would soon be occupied by glaciers, as they were in former times. So if winter conditions could be made permanent on the great water-shed of the Canadian highlands, and the water of the St. Lawrence, the Mississippi and Red Rivers were retained in the form of snow and ice, glaciers would fill again the lake-basins, override the highest summits, and cover with an ice-sheet all the old glaciated areas.

Even if the evaporation from adjacent seas were somewhat diminished by the cold, that would not change the result, though it would prolong the time. The evaporation from the ice-cold oceans in the regions surrounding the north and south poles is now sufficient to produce continental glaciers in Greenland and on the Antarctic Continent, and it requires no argument to show that like conditions would produce like results in what is now the temperate zone.

On the theory that increased evaporation and precipitation would cause an extension of glaciers, and as an illustration of their local origin, it has been suggested that the water with which the dry regions of our Western Territories were once abundantly supplied produced the glaciers of which we find traces on the adjacent mountains, and the exhaustion of the water caused the disappearance of the glaciers. We shall see, however, that this is a speculation which is as yet sustained by no proof. It is well known to all geologists that the interior of the North American Continent has been occupied by a succession of great fresh-water lakes, extending in time from the early Eocene to and through the Quaternary. The history of these lakes has been admirably worked out by King, Gilbert, and Russell. Those of the Tertiary were numerous and broad, providing ample evaporating surfaces, but so far as we know they contributed nothing to the formation of glaciers—which could not have existed, indeed, under the warm sun