Shaler.] 290 [December 16, 



It is not to be denied that simultaneous ice caps would still tend to 

 drag the oceans towards both poles, but the amount of water taken 

 from the sea under these conditions would be so great that we 

 should rather expect to find signs of elevation than subsidence. Fur- 

 thermore, it is worthy of note, that the re-elevation of circumpolar 

 regions now going on, doubtless the same in origin as the other ele- 

 vations sought to be explained by this theory, is distinctly not tracea- 

 ble to a readjustment of the centre of gravity. 



4. Sinking of the sea level through the abstraction of water stored in 

 glacial sheets. 



This action connects itself in a general wa^' with that just dis- 

 cussed. It is unquestionably a true cause, though its value is not eas- 

 ily determined. Both the depth of the ice and the area it covered 

 are still questionable points ; assuming, however, that one-fourth of the 

 earth's surface is the largest amount that can at any time be ice 

 covered, and assuming that a mile is the average thickness of the 

 sheet, then, making allowance for the space occupied by the land in 

 the regions not covered with ice, the depression of the sea level will 

 be something like twelve hundred feet. Prof. Benjamin Pierce, has 

 called my attention to the fact that this depression would decrease 

 the temperature of any point as much as if it had been subjected to 

 positive elevation of the same amount. There are some important 

 biological results following from this depression of the shore level. 

 In the first place, if all the life of the glacial seas was removed to 

 depths twelve hundred feet below the present level of the sea, or in 

 other words, if the shore line was to any considerable extent removed 

 to the seaw r ard of its present position during the height of the period, 

 we can well account for the fact that we have failed to find in the 

 region bordering on the tropics the fossil remains of the fauna driven 

 from the circumpolar region by the advent of the glacial period. 



5. Change in the jjosition of the axis of rotation of the section from 

 the centre of the sea to the centre of the continent. 



I have already called attention to this action, whereby, while the 

 continent is continually rising and the sea floor continually sinking, the 

 different positions of the rotation point of the movement might give us 

 the three phenomena : of gain of land, gain of sea, and apparent 

 steadfastness, according as the pivot point happened to lie to the sea- 

 ward of the shore, on the land, or just at the coast. The real na- 

 ture of the movement may be all the while one of constant elevation 

 and yet give us these three effects on the shore line. 



