90 THE AGE OF MAMMALS 



lignite; the proofs of prolonged overflow or lagoon conditions are found in the 

 great horizontal bands known as the "white layers." As will be more fully 

 described below, the vast building materials of these deposits were partly 

 derived by erosion from the surrounding mountain ranges, but chiefly by 

 sediments of volcanic dust which have consolidated into what are known 

 as tuffs. 



These great facts of western physiography may be summed up as follows: 

 (1) the axes of the mountain ranges were the same as at present; the ranges 

 themselves, though not actually as high, were probably higher in relation to 

 the surrounding country than at present because we must allow for a long 

 period of erosion. (2) The Eocene drainage systems were also broadly the 

 same as the modern, namely, the systems of the Colorado River, the Arkan- 

 sas River, the Big Horn branch of the Missouri River, and the Columbia 

 River. In details, however, the drainage systems have certainly been modi- 

 fied by uplift and erosion. (3) The deposits all lie in the same great mountain 

 basins or mountain valleys in which they were originally collected. Owing 

 to the proximity of volcanic peaks, ash and other fine eruptive materials 

 contributed very largely and in some basins almost exclusively to these 

 Eocene and Oligocene deposits of the Mountain region. (4) Except close 

 to the mountain foothills, as, for example, in the Wasatch of the Big Horn 

 basin (BH), there has been comparatively little post-Eocene disturbance, 

 because these deposits are still horizontal or at gentle angles with their 

 original horizontal position. (5) Surrounding mountain ranges were inter- 

 spersed with active volcanic peaks; the upper Colorado River basin especially 

 was surrounded by a circle of volcanoes which poured out their lava and widely 

 distributed ashes. (6) The mammalian life of the mountain region is largely 

 that of plateaux, uplands, and elevated basins, of streams and lake borders, 

 of hillsides, and more or less of forests. There are also evidences of arid peri- 

 ods in which great stretches of the sandy flood plains were desiccated during 

 the dry season of the year, and afforded a favorable basis for the evolution of 

 mammals with slender or cursorial types of limbs and feet, as of the smaller 

 horses. 



Volcanic materials. — A very large part of the basin deposits of the 

 Rocky Mountain Region which for years were described as consisting of 

 gravels, sandstones, and clays, are now found to consist largely and in some 

 places exclusively of volcanic ash materials. As early as 1876 Clarence 

 King ' recognized volcanic ash strata in the lower Eocene Wasatch of west- 

 ern Wyoming underlying the true Coryphodon Zone. Merrill and Peale in 

 1885-1886 determined the volcanic ash origin of the Bozeman Lake deposit, 

 Gallatin County, Montana. Peale ■ prophetically observed: "Again, do 

 not these volcanic materials which must have faUen in showers over a large 

 extent of country — accumulating in some cases in beds 40 to 90 feet thick — 



' King, C, Amer. Jour. Sci., Ser. 3, Vol. XI, 1876, pp. 478-480. 

 2 Peale, A. C, Science, Vol. VIII, Aug. 20, 1886, p. 163. 



