2l6 



THE GUIDE TO NATURE 



hundreds of thousands of years, ago, 

 there was a change of cHmate, by which 

 the mean temperature at least of large 

 areas of the earth's surface came to be 

 somewhat lower than it had been in pre- 

 vious ages, and lower than it is at pre- 

 sent. The cause of this change of climate 

 is not certainly known, but the most 

 probable theory is that it was due to a 

 diminution in the amount of carbon 

 dioxide in the atmosphere. It would 

 not require a change of many degrees 

 in the mean temperature to prevent the 

 winter snowfall from melting away 



St. Lawrence valley. If the mean tem- 

 perature were reduced a little, a larger 

 share of the precipitation woidd be 

 snow, a smaller share would be rain. 

 In the highlands between the St. Law- 

 rence and Hudson Bay, the mass of 

 snow accumulated in the Glacial period 

 to a thickness of thousands of feet, and 

 gradually extended itself in all direc- 

 tions. The weight of the mass of snow 

 gradually consolidated it into ice, and 

 produced a slow, creeping movement 

 of that ice outward from the center of 

 the area. At the extreme of s^laciation 



FIG. 5. BOULDERS OF DISINTEGRATION, NEAR BUTTE. MONTANA. 



entirely in the following summer in 

 considerable areas of Canada and 

 northern New England. If a little of 

 the winter's snow remains unmelted 

 through the following summer, each 

 winter will add somewhat to the accu- 

 mulation, and the mass of snow will 

 increase both in depth and in horizontal 

 area. The snowfall is now very heavy 

 in the track of the cyclonic storms 

 which move northeastward down the 



the thin edge of the great ice sheet 

 had crept southward in the eastern 

 United States to about the latitude of 

 40°. Farther west, where the air is 

 drier and the precipitation less, the 

 southern boimdary of the ice sheet was 

 hundreds of miles farther north. Several 

 times during the long history of the 

 Glacial period the climate became war- 

 mer, and the edge of the ice sheet melt- 

 ed back for scores or hundreds of miles, 



