G. F. Wright— Unity of the Glacial Epoch. 359 



be sufficient, and this is much less than we know has occurred 

 in central Europe since Miocene times. Miocene shells are 

 found on the Alps and Pyrenees at an elevation of more than 

 10,000 feet. An elevation of our glaciated region in its 

 northern parts of -3,000 feet is therefore not out of analogy 

 with the movements during contiguous eras. 



Supposing therefore the elevation of the region south of 

 Hudson Bay to amount to 3,000 feet, this would provide the 

 conditions necessary for the beginning of an ice age. But 

 when such a career is once started it is difficult to imagine how 

 it can stop, for the accumulation of ice at once lowers the 

 temperature and adds to the elevation of the surface of the 

 plateau on which it rests; thus tending to increase the depres- 

 sion of the land. The ice piles up and makes an elevation of 

 x- its own, additional to that of the land, and overloads it with 

 its own weight. 



There is certainly much plausibility in the supposition that 

 the subsidence which accompanied the glacial period, and 

 from which the continents have not yet fully recovered, was 

 in part due to the weight of the ice piled up over the glaciated 

 area. The conditions were then unique in the geological 

 history of the earth. About 4,000,000 square miles of terri- 

 tory in the northern part of North America and about half 

 that amount in. Northern Europe were covered with ice aver- 

 aging, probably, three-quarters of a mile in depth, making in 

 all 5,500,000 cubic miles of ice whose weight had first been 

 abstracted from ocean beds, thus relieving the pressure there, 

 and then centered over a restricted area at the north. The 

 relative extent of this disturbance of equilibrium can be 

 appreciated only by comparing this mass of ice with the solid 

 contents of that portion of the continent which is above ocean 

 level. The mean elevation of North America above the sea 

 is estimated by Wallace (Island Life, p. 205) to be 748 feet, 

 while the total area is less than 9,000,000 square miles. This 

 would give about 1,300,000 cubic miles of land in North 

 America above the water level, as against 3,000,000 cubic 

 miles of ice over the northern half of the continent during 

 the climax of the glacial period. Taking the specific gravity 

 of the rocks as two and a half times that of ice, the total 

 weight of ice piled up over British America and the Northern 

 United States, at the time referred to, was nearly equal to 

 that of the whole land surface of the continent which is above 

 sea level. That this enormous change in the distribution of 

 weight and pressure on the surface of the earth should occur 

 without any effect on the configuration of the globe is difficult 

 to believe. When therefore we find that extensive changes in 

 level seem actually to have been conflated with this loading 



