286 THE POPULAR SCIENCE MONTHLY. 



rock-surfaces extend beneath the sea ; but this, as we have seen, 

 proves no such thing. 



Dana bases his statement that the northern portion of our conti- 

 nent was highest in the Ice period on the system of deep, now-buried 

 channels, by which its surface was once furrowed, and upon the fiords 

 which fringe the northern coast ; but, as elsewhere stated, we have 

 no proof that all, or nearly all, this erosion was not effected previous 

 to the Glacial epoch. Reviewing all the facts that have been cited, 

 we can at least say that the indications of elevation are not nearly so 

 well marked in the Quaternary as in the Tertiary ; and the evidence of 

 such elevation as would shut out the tropical currents from the Arctic 

 Sea in the Quaternary age is wholly wanting. 



In the Champlain epoch the northern land was greatly depressed, 

 as we learn from the fact that the clays containing marine shells are 

 found on the present land at a constantly-increasing elevation as we 

 go toward the north. About New York the Champlain clays reach 

 from 50 to 100 feet above the ocean-level ; on Lake Champlain they 

 are 400 feet, at Montreal nearly 500 feet, at Labrador 800, in Bar- 

 row's Straits 1,000, and at the extreme point reached by the Polaris 

 Expedition, on the coast of Greenland, 1,800 feet above the sea 

 (Bessel). 



On the European coast of the Atlantic we have proof of an eleva- 

 tion of the land during the Tertiary, and a subsidence in the Quater- 

 nary, similar to those described above. Hence we may infer that in 

 the Champlain epoch the topography of the arctic regions was just 

 that which would be favorable for the transfer by ocean-currents of 

 the heat of the tropics to the arctic, and a prevalence over the arctic 

 regions of a warm climate. But it must be said that all the shells 

 found in the Champlain clays, from Lake Champlain to Greenland, 

 are of a decided boreal character, which indicates that during the en- 

 tire deposition of that formation a climate scarcely warmer than that 

 of Greenland prevailed from New England northward. 



If it is true that the Glacial epoch was one of elevation at the 

 north an elevation of the land much greater than the present the 

 change to the depressed condition of the Champlain epoch, when the 

 sea stood from 1,500 to 1,800 feet higher on the coast of Greenland 

 than it now does, must have been comparatively sudden ; and if, as 

 has been asserted, the depression of the Champlain epoch was com- 

 mon to the whole northern hemisphere, it could have been effected 

 only by a great change in the figure of the earth, or by a flow of the 

 ocean-waters into the polar regions, such as has been suggested by 

 Adhemar and Croll. These writers hold the view that the effect of 

 the extreme cold of the Glacial period was to form an ice-cap some 

 miles in thickness over the arctic regions, and that this ice-cap moved 

 the centre of gravity of the earth toward the pole, so that the oceanic 

 waters flowed into this hemisphere and thus elevated the sea-level. 



