330 SURFACE GEOLOGY. 



Po rivers it was at least 400 feet. If we reflect upon the widespread 

 changes of sea-level that marked the glacial period, occurring only where 

 they would be produced by taking water from the sea to form the ice- 

 sheets, and by gravitation through their influence, and if we compare 

 these recent simultaneous changes with the general stability of the con- 

 tinents, it seems reasonable to attribute them to movements of the sea 

 rather than of the land. 



In North America the southern edge of the ice-sheet varied from 38° 

 to perhaps 50° north latitude. Nearly all of the continent north of this 

 line, with portions of the sea next to the coast, the archipelago farther 

 north, and much of the Arctic ocean, Hudson's and Baffin's bays, and 

 Greenland, were probably covered by ice in the glacial period. This 

 would be about one twenty-fourth part of the whole area of the globe. 

 In the eastern hemisphere, Europe and Asia were apparently overspread 

 by ice as far south, on the average, as to 50° north latitude. The North 

 and Baltic seas, and a considerable part of the Arctic ocean, are to be 

 added, making an area, as before, equal to about one twenty-fourth part 

 of the earth. The glacial sheets of the antarctic continent and adjacent 

 ocean, with Patagonia and its sea-border, were probably equal to each of 

 the foregoing, so that in all about one eighth of the earth's surface was 

 covered by ice. If a slope of one half of a degree is needed to cause 

 the motion of these sheets of ice, an estimate of four miles for their aver- 

 age depth does not seem to be too great. The removal of the water thus 

 taken from the sea and stored up in accumulations of ice would lower 

 the surface of the ocean more than a half mile. 



The effect of the ice-caps to draw the sea towards the poles remains 

 to be considered. Because the ice was limited to high latitudes, its influ- 

 ence to raise the ocean over these areas would be much greater than if 

 the same amount of ice had been spread in a thin covering, reaching, with 

 gradually decreasing depth, to the equator. It may therefore be near the 

 truth, to consider the effect in gravitation over glaciated regions to be 

 the same as would result from an increase of the polar diameter by twelve 

 miles of ice. This would be massed, as we have seen, in the proportion 

 of two to one about the north and south poles, the greater part being 

 accumulated in the northern hemisphere; still, the effect upon the sea- 

 level would be nearly alike about both poles, in the same way that the 



