Page:The Journal of geology (IA journalofgeology21894univers).pdf/401

but only to a comparatively small amount, from 200 to 500 or 600 feet, after the departure of the ice-sheet. In Scandinavia, according to the investigations of Baron de Geer, the postglacial uplift has varied from a minimum of 100 feet or less at the southern extremity of Sweden, to a maximum exceeding 1,000 feet in the central part of the peninsula.^[1] Likewise in South America, along a distance of 1,200 miles, from the Rio Plata to Tierra del Fuego, the land has been elevated since its glaciation, the general extent of this movement in Patagonia, as observed by Darwin, being between 300 and 400 feet.^[2]

The special case of an epeirogenic movement progressing like a wave, which it is the purpose of this paper to consider, is this latest, moderate uplift of North America, and especially of its central belt comprised in the Mississippi and Nelson river basins, from its depression at the close of the Glacial period. While the ice-sheet was retreating, this great area was rising as fast as its burden was removed. Close upon the wasting ice-border there followed a wave of permanent uplift of the land on which it had lain. First the loess district along the Mississippi and the upper part of this basin were elevated; next, the southern half of the area of the glacial lake Agassiz; later, its northern half; and last of all, the country enclosing Hudson bay, with which also was probably associated, as very late in its uplift, the region of the great Laurentian lakes, including lake Champlain, and of the Ottawa and the St. Lawrence. From south to north and northeast the wave of elevation advanced, and, according to Dr. Robert Bell, the rise of the land has not yet ceased about James and Hudson bays, where, in the central part of the glaciated region, we must suppose that the ice-sheet had its greatest thickness and was latest represented by lingering remnants. Having thus outlined our theme, let us return and look more