Page:Popular Science Monthly Volume 15.djvu/326

312 we learn from various sources, at the close of the Cretaceous the widespread sea of that age was withdrawn from the interior of the continent, and all the interval between the Sierra Nevada and the Canadian highlands became a land-surface; while in the lower valley of the Mississippi, and on the Gulf and Atlantic coasts of the eastern half of the continent, the sea stood higher than before or now, for marine Tertiary strata form a broad marginal belt reaching around the old land from New York to and up into the Mississippi Valley. In the region of the Plains, the Rocky Mountains, and the Great Basin, however, we find no marine Tertiaries, but abundant evidence that, instead of the former sea-surface, a broad continental area stretched from the Arctic Ocean southward through and beyond the Territories of the United States. This continent was marked by few bold topographical features, since the Rocky Mountain system was then slowly growing, and had attained nothing like its present magnitude. The surface was, however, varied with low mountain-chains, broad savannas, strongly flowing rivers, and a series of fresh-water lakes, which in magnitude far exceeded any now on the earth's surface. The climate was mild and genial even to the North Sea, and the land was clothed with a vegetation more luxuriant and varied than that which it now bears. Of the magnitude of its forest growth we have evidence not only in the abundant remains of trunk and leaf and fruit imbedded in the old lake sediments, but in the scattered remnants of its former grandeur seen in the gigantic conifers of California, and in the cypress, magnolias, sweet gums and sycamores, which are the pride of our Eastern forests. This fertile land also sustained a fauna corresponding in richness and interest to its flora; for in the Tertiary the gigantic reptiles of the Mesozoic were succeeded by herds of mammals which far surpassed in numbers, size, and variety of species, any mammalian fauna now living. Their remains have been exhumed by thousands from the old lake-beds, where, in the long lapse of ages, they had been borne by river-floods and entombed. Thus were formed the vast charnel-houses from which Leidy, Marsh, and Cope, have drawn the treasures they have exhibited to the admiring scientific world. One after another of those great Tertiary lakes were created by topographical changes which established hydrographic basins, and, in turn, by the cutting down of their outlets, their beds were first made dry land, and afterward deeply cut by the many-branched draining streams, until they have formed the Mauvaises Terres or "Bad Lands" of the West.

The Tertiary deposits, then, cf the region west of the Mississippi are fresh-water sediments, chiefly the immediate wash of the land, containing fossils which represent not only the fishes and turtles which were their aquatic inhabitants, but the flora and fauna that lived upon their banks.

On the west, however, this lake country was bordered by a chain of volcanoes, which had from time to time their paroxysms of activity,