CENOZOIC ERA: AGE OF MAMMALS 575 



over a considerable territory, but these later gave way to brackish 

 or fresh-water swamp conditions. The physical history during the 

 latter part of the period is one of persistent but frequently interrupted 

 submergence, in which the alternation of many coal beds (some work- 

 able) with deposits of fine shale and coarse sandstones indicates that, 

 during this great subsidence, the depth of the water frequently 

 changed. At times the sinking proceeded more rapidly, and the 

 deepened water was then filled with sediment, " until the tide-swept 

 flats became marshes and, for a time, vegetation flourished vigorously 

 in the moist lowlands" (Willis), this rotation being repsated inter- 

 mittently. This condition is believed to have prevailed in Alaska, 

 western Oregon, and the Great Valley of California. Most of the coal 

 of the west cOast belongs to this epoch, making this the " Eocene 

 Carboniferous" of the west. In the later Eocene, elevation and ero- 

 sion, accompanied by volcanic outbursts and extensive lava flows, 

 occurred in Oregon and Washington. The presence of Atlantic 

 species in the marine deposits shows that an oceanic connection, 

 probably in the Central American region, was in existence for a time. 

 Western Interior. — The Eocene deposits of the western interior 

 (Fig. 529), with the exception of a few small areas in Colorado, are con- 

 fined to the region between the Sierra Nevadas and the Rocky Moun- 

 tains. It is thought that the region under discussion was not greatly 

 elevated above sea level, although the summits of the mountains 

 probably stood sufficiently high above the general level of the plains 

 to permit the vigorous erosion which was in progress during the epoch, 

 and which furnished the waste to form a great thickness of sediments. 

 The mountains and hills, formed by folding, by faulting, by warping, 

 and by volcanic debris, inclosed basins and valleys in which the 

 streams deposited the sediments obtained from the steep slopes of 

 the higher lands. These sediments were deposited partly in lakes 

 and partly in alluvial fans in front of the valleys which the streams 

 had cut in the mountain slopes. The most important deposits, 

 however, were laid down in flood plains, in deltas, and in swamps. 

 From time to time the. area of deposition shifted, because of the fill- 

 ing up of old basins or the warping of the land. Lakes were also in 

 existence, the most famous being one in Wyoming in which the Green 

 River formation occurs, consisting of impure limestone and thin, 

 fissile calcareous shales, often as thinly laminated as paper. Be- 

 tween the leaves of these shales remains of plants, insects, and fishes 

 are beautifully preserved, but no remains of mammals are found, 



CLELAND GEOL. — 37 



