Page:The Journal of geology (IA journalofgeology21894univers).pdf/237
believe to be without foundation. Its fallacy appears when the quantitative elements of the problem are considered.
Let it be assumed that northward elevation was the cause of the cold climate which made the development of the Pleistocene ice-sheet possible. Let it be assumed further (and this is the assumption most favorable to the doctrine here opposed), that the elevated region was in isostatic equilibrium at the time the ice began to accumulate. Let it be assumed also, that the average specific gravity of the mass of snow and ice of the ice-sheet was one-third that of the rocks of the earth's crust. On the doctrine of isostasy, depression should have accompanied the accumulation of snow and ice. When the central part of the snow-field had a depth of 300 feet, the maximum depression which it could have caused, under the assumed conditions, was 100 feet. At the minimum, therefore, the surface of the central part of the ice-field must have been 200 feet higher than the surface of the land before the ice-field formed. Nearer the margins of the ice-field, where the ice was thinner, both the depression of the land surface and the accompanying elevation of the snow surface would have been less; but each point of the surface of the snow-field must have been higher than the corresponding point of the surface of the land at the time the ice began to accumulate, and the temperature at all points must have been correspondingly reduced. Instead of being ameliorated by the depression of the land surface, the very conditions which brought about this depression were causing the climate to become progressively more severe. When the ice had attained a thickness of 3,000 feet, it might have occasioned a maximum depression of the subjacent land surface to the extent of 1,000 feet, and therefore a minimum elevation of the ice surface at the same point, to the extent of 2,000 feet. While, as before, both the depression of the subjacent land surface and the correlative elevation of the surface of the ice-sheet would have been less near the margins of the snow-field than at its center, it still remains true that each point of the entire surface of the ice must have been higher than the corresponding point of the surface of the land at