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 
