3 i2 THE POPULAR SCIENCE MONTHLY. 



we learn from various sources, at the close of the Cretaceous the wide- 

 spread sea of that age was withdrawn from the interior of the conti- 

 nent, and all the interval between the Sierra Nevada and the Canadian 

 highlands became a land-surface; while in the lower valley of the Mis- 

 sissippi, 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 fea- 

 tures, 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 vegeta- 

 tion more luxuriant and varied than that which it now bears. Of the 

 magnitude of its forest growth we have evidence not only in the abun- 

 dant remains of trunk and leaf and fruit imbedded in the old lake sedi- 

 ments, 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 estab- 

 lished 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, con- 

 taining 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, 







