. The age of mammals in Europe, Asia and North America. Mammals, Fossil; Paleontology. 278 THE AGE OF MAMMALS Thus, as shown in the accompanying sections of the Pine Ridge (Fig. 138), of Sioux County, Nebraska (P^ig. 99), the Arikaree, 'Upper Harrison,' and 'Upper Rosebud' beds, while geologically continuous with the Harrison, and showing no unconformity or other evidence of a break in time, yet do lack some of the mammals found in the Harrison and do. By permission of the Geological Survey. Fig. ? Diagrammatic section of the Lower Miocene. Taken near Harrison, Nebraska. After Peter
. The age of mammals in Europe, Asia and North America. Mammals, Fossil; Paleontology. 278 THE AGE OF MAMMALS Thus, as shown in the accompanying sections of the Pine Ridge (Fig. 138), of Sioux County, Nebraska (P^ig. 99), the Arikaree, 'Upper Harrison,' and 'Upper Rosebud' beds, while geologically continuous with the Harrison, and showing no unconformity or other evidence of a break in time, yet do lack some of the mammals found in the Harrison and do. By permission of the Geological Survey. Fig. ? Diagrammatic section of the Lower Miocene. Taken near Harrison, Nebraska. After Peterson. contain some new mammals not found below. There are proofs of a long interval of time, of several extinctions, and of some quite profound changes of environment and of evolution. Further exploration may modify this artificial line, and either lessen or intensify it. Ancient Physiographic Conditions The Plains Region Geologic conditions. — The conditions of deposition of these upper beds east of the Rocky Mountains indicate the continuance of the same physio- graphic features, namely, of great flood plains with a gentle slope in an easterly direction, traversed here and there by river channels containing coarser deposits. The typical Arikaree Formation of Darton,^ as observed at Pine Ridge in northern Nebraska in 1899, is in the same region as the 'Rosebud' of Matthew. He observes (p. 176) that a large portion of the high lands of western Nebraska and southeastern Wyoming extending from Pine Ridge is occupied by the sands and soft sandstones of the Arikaree Formation. It attains a thickness of over 800 feet in southeastern Wyoming, and formerly extended far up the slopes of the mountains to the north and west. As thus defined, the Upper Harrison of Hatcher, and Upper Rosebud correspond to the upper part of the Arikaree Formation of Darton. ' Darton, N. H., Preliminary Report on the Geology and Underground Water Resources of the Central Great Plains. Geol. Sun., Prof.
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