660 HISTORICAL GEOLOGY 



layers (much as the age of a tree is determined by the rings), 

 it is estimated that Stockholm was covered with ice only nine 

 thousand years ago, and that the glaciers withdrew at a rate of 800 

 feet a year. The ice is believed to have receded from Ragunda, 

 Sweden, only 7000 years ago. 



Causes of Glaciation 



Numerous theories have been offered to account for the refrigera- 

 tion of the climate which resulted in the Glacial Period, each of 

 which has elements of probability, but none of which, as at present 

 worked out, is perfect. In the consideration of all of the theories 

 discussed below, it should be borne in mind that the appearance of 

 an ice sheet does not necessarily imply an extremely low average 

 temperature. It has been estimated that a fall of 3 F. in the average 

 temperature of the Scottish Highlands, and a fall of 12 in the Lauren- 

 tian region of Canada would result in a glacial period for these re- 

 gions. 



1. Elevation. — The explanation of glaciation which naturally 

 suggests itself is that the refrigeration was due to a great elevation 

 of the land in the northern hemisphere, which so reduced the tem- 

 perature that snow accumulated to form glaciers as it does now on 

 high mountains. Those who hold this theory point to the evidence 

 of an elevation of several thousands of feet as shown by the fiords 

 on northern coasts. The objections to the theory are (1) that it is 

 probable maximum elevation and maximum glaciation did not coin- 

 cide in time ; (2) that the elevation was not as great as once sup- 

 posed; (3) that glaciation not only occurred in the northern hemi- 

 sphere, but that mountain glaciers throughout the world were more 

 extensive than now. (4) Moreover, this hypothesis would require 

 a great elevation for the glacial stages and a corresponding depression 

 for the interglacial. 



2. Astronomical. — A theory which at one time had wide accept- 

 ance was offered by Croll and is known as " Croli's hypothesis/' 

 It is based upon (1) the variation in the eccentricity of the earth's 

 orbit as a result of which the relative length of the summer and 

 winter seasons changes (Fig. 574). When the eccentricity is great- 

 est the earth is 14,000,000 miles farther from the sun in the one 

 season than in the other. (2) By means of the precession of the 

 equinoxes the winter of the northern hemisphere, which now occurs 

 when the sun is nearest the earth (perihelion) (Fig. 574 A), is 



