336 RECENT CHANGES OF LEVEL 



by the attraction of the mass of ice about either pole ; but it is clear that inasmuch as the 

 seas do not rise under the influence of the masses of the continents against which they 

 rest, at least more than a trifling amount, so the water could not rise against the polar 

 ice caps to any great height. The double polar ice cap, even were it a mile thick, could 

 not affect the gravitation of the sea more than the high lands of Western South America. 

 Moreover, the fact that in high northern countries some time elapsed after the disappear- 

 ance of the ice cap before the re-emergence of the land took place, while on the theory of 

 disturbed gravitation it should have disappeared jxiri passu with the waning of the ice, is a 

 strong argument against the sufficiency of this explanation. Probably the most insupera- 

 ble objection which can be made against this hypothesis of depression through a change of 

 the centre of gravity arising from the magnitude of the ice sheet, is found in the fact that 

 the depression does not increase with regularity throughout the whole of the Northern 

 Hemisphere, which it should have done if this view be correct. The increase from a few 

 feet (not exceeding fift}') along Long Island Sound to three hundred on the coast of Maine, 

 is excessive. This rate, if kept up, as it must have been if tliis hypothesis be true, would 

 have made the depression at the poles many miles of depth. 



In view of these arguments I find myself compelled to al^andon this view concerning the 

 origin of the glacial depression. In seeking another explanation of the phenomenon, I have 

 endeavored to arrive at something which should be easily connected with the general facts 

 of continental and other mass movements of the earth's crust. In pursuing this object the 

 following opinions have been forced upon me. 



The constant movements of sea and land show clearly that the surflice of the earth and 

 the solid matter, for a consideraljle depth, are subject to movements which vary mucli in 

 direction and intensity. At first sight, however, it would seem as if this variety of move- 

 ment was fiir more considerable than it is in f\ict. The following considerations will serve 

 to limit the phenomena in tlie range of its action in an important way. 



As the sea is the region of constant deposition, and the land of constant erosion, there 

 must be in the long run quite constant upward movement of the land areas and depression 

 of the floors of earth beneath the seas. For instance, the region of the Ohio valley was 

 near the sea level in the silurian and carboniferous times, and is a few hundred feet above 

 it at the present day, notwithstanding the constaiit erosion which has affected it nearly ever 

 since. We cannot reasonably reckon the time wdiicli has elapsed since the coal period at less 

 than twenty million years. Now the rate of wear on this region, as shown by the dischai-ge 

 through the Mississippi River, will carry away about one foot in seven thousand years. 

 This may be reduced to one foot in ten thousand years, if we would keep within bounds. 

 But even this slow rate will retpiire a steady rise of at least twenty-five hundred feet since 

 the close of the carlxmifeious period, relatively a very modern period. Further to the 

 northward, the Laurentian Hills, judged by their age, must have lost several times as much 

 height, even supposing they waste only at the rate of the plain, which is far Ijelow the 

 south. Tbis class of facts entitles us to suppose that the lands are, on the whole, constantly 

 rising. On the other hand, all (he known facts, such as the continuous deposition of strata 

 to the amount of thousands of feet in waters always sliallow, the increasing evidence of 

 ancient zoological barriers in the sea, under circumstances Avliicli require us to suppose that 

 depth of water was the obstacle to the exchange of life ; and other facts which cannot be 



