Page:Popular Science Monthly Volume 9.djvu/308

286 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 continent 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 Barrow'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 elevation of the land during the Tertiary, and a subsidence in the Quaternary, 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 entire 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 common 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.