XORTH AMERICA IN THE ICE PERIOD. 7 



growth of trees and smaller plants. The fir-forests here meet the 

 snow-hanks 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 precipita- 

 tion remains, the snow-fall remains, but the glaciers are gone. Here 

 we have just the conditions most favorable to the formation of gla- 

 ciers 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 in- 

 creased evaporation and precipitation, it is evident that no glaciers 

 would form ; but w T ith 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 iu 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 dimin- 

 ished 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 pro- 

 duce continental glaciers in Greenland and on the Antarctic Continent, 

 and it requires no argument to show that like conditions would pro- 

 duce 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 ori- 

 gin, 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 sus- 

 tained 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 sur- 

 faces, but so far as we know they contributed nothing to the formation 

 of glaciers — which could not have existed, indeed, under the warm sun 



