DEPTH OF THE ICE DURING THE GLACIAL PERIOD. 201 



have been less than that indicated by the evidence in JSew 

 England. Over southern Ontario and Michigan, and over 

 the larger part of Wisconsin, Minnesota, northern Illinois, 

 and Iowa, the ice must have been thousands of feet in depth, 

 or it never could have pushed southward to the latitude of 

 Cincinnati, Louisville, and St. Louis. 



The uncertainties attending this mode of calculation are, 

 however, very great, and it can be taken only for approxi- 

 mate results. In the Alps the lowest mean slopes down 

 which glaciers move are 2f° to 3°, or about 250 feet to the 

 mile. But, as Professor Dana notes,* the thickness of the 

 ice there is not over 500 feet. Mathematicians are not able 

 to deal successfully with the problems of friction in viscous 

 bodies. How such a body will behave in greatly increased 

 masses can be determined only by experiment. In Green- 

 land, where the thickness of the ice more nearly approaches 

 that of the ice-sheet formerly covering the northern part of 

 the United States, Jensen found the slope of the Frederik- 

 shaab Glacier to be 0° 49', or about seventy-live feet a mile; 

 while Helland found the slope of the Jakobshavn Glacier to 

 be only 0° 26', or about forty-five feet to the mile.f This 

 latter slope of the surface of the continental glacier would, 

 if continuous, make the thickness of the ice 10,000 feet over 

 northern New England, and about 11,000 feet over Lake 

 Erie, while the depth of the ice in this calculation over the 

 region north of Lake Huron and Lake Superior, from which 

 certain bowlders in Kentucky came, would be nearly 30,000 

 feet, since the distance moved is 600 miles or more. 



Upon the supposition that the slope from the front to- 

 ward the interior was but half a degree, Croll estimates that 

 the depth of ice at the south pole, at the center of the Ant- 

 arctic Continent, must be as much as twelve miles. This is 

 on the supposition that the diameter of the continent is 2,800 

 miles. The same rate of calculation would, according to 



* " American Journal of Science," vol. cxxvi, 1883, p. 348. 

 \ " Meddelelser om Gronland," 18*79, and "American Journal of Science," 

 vol. cxxiii, 1882, p. 364. 



