1874.] 289 [Shaler. 



cations of the average shore level upon the whole earth. 1 One of 

 these changes is in a determined direction. The average erosion of 

 the land at present is probably not far from one foot in five thousand 

 years. With the present areas of land and sea this would raise the 

 sea level one foot in about fifteen thousand years. This seems a slow 

 rate, but allowing only sixty-six feet in a million years, and 20,000,000 

 since the beginning of the carboniferous (probably far too short a 

 time), we have a total of 1320 feet. It is to be noticed that this 

 change is constant and in one direction, so that the other vascillating 

 changes will not overcome it. It will cause a steady accumulation of 

 height through all accidents of elevation and subsidence. 



3. Change in the centre of gravity of the earth arising from the form- 

 ation of an ice cap on either polar region. 



This hypothesis of Adhemar accounts for a depression of cir- 

 cumpolar and neighboring regions by the change in the position of 

 the earth's centre of gravity, caused by the accumulation about either 

 pole of a large part of the oceanic waters in the form of an ice cap. 

 The value of this cause cannot be determined. It depends for its 

 existence on the hypothesis that the ice cap is formed in succession 

 at each pole, and is liable to the correction we must apply for the subsi- 

 dence of the sea caused by the removal from it of a large part of 

 the water. The evidence goes to show that the last glacial period at 

 least was simultaneous in the two hemispheres ; this is so entirely 

 against this hypothesis that we are not certain that it has been a 

 true cause. Furthermore the depression produced during the last 

 glacial period was so unequal in regions under the same parallel of 

 latitude that there must have been other causes 'of depression oper- 

 ating at that time. A depression of one thousand feet in Wales be- 

 comes almost nothing in France. The irregularity in the amount of 

 subsidence is almost as great in this country as in Europe, and goes far 

 to disprove the equality of the subsidence under the same parallels, 

 which is essential to the truth of this theory, and when taken in con- 

 nection with the evidence of simultaneous glaciation in both hemi- 

 spheres may be regarded as putting this hypothesis far within the 

 bounds of improbability. 



1 My attention was, I believe, first called to this possible cause of change of 

 level, by my friend Professor J. P. Leslie. I am not aware that he has published 

 this conclusion ; but as it is indisputable, and is important to the matter treated of 

 here, I venture to make use of it. As it is a recollection of ten years or so, I may 

 not have accurately represented his idea. 



PROCEEDINGS B. S. N. H. — VOL. XVII. 19 MARCH, 1875. 



