26 



CENOZOIC MAMMAL HOEIZONS 



coarser materials by streams into the so-called sandstones, while the 

 finer materials, constituting the so-called clays, are actually tuffs. 

 Proofs of temporary lacustrine conditions, or of prolonged high water 

 on base-level, are found in the very widely extended so-called white 

 layers containing calcite and flint; these divide theBridger formation 

 into five levels (A, B, C, D, E), each characterized by distinctive 

 specific forms of mammalian and reptilian life. These levels dem- 

 onstrate periodic risings of the water level in this basin. 



10. As the fossil mammals which all these Eocene mountain depos- 

 its contain are carefully compared and studied, we nearly, if not quite, 

 demonstrate another great fact, namely, that these deposits were 

 successively formed, in one basin after another, throughout the 

 Eocene period; in a number of cases, fortunately, there was a, time 

 overlap — in other words, before one deposition closed another began. 

 When fully explored they will thus afford a nearly continuous history 

 of the vertebrate life of the Mountain Region during the Eocene and 

 Oligocene epochs. 



GEOLOGIC HISTORY OF THE GREAT PLAINS DEPOSITS 

 OF THE OLIGOCENE. TO LOWER PLEISTOCENE. 



Extent. — The Oligocene to Pleistocene deposits immediately over- 

 lie the various divisions of the Cretaceous and form the surface of the 

 plains at different points from 200 to 300 miles east of the Rocky 

 Mountains, from British Columbia on the north to the Mexican plateau 

 on the south, with a combined maximum thickness of about 2,000 

 feet. Their central area is best shown in Barton's preliminary geo- 

 logic map of the central Great Plains." 



History of opinion as to mode of deposition.^ — The lacustrine-origin 

 theory as to the Great Plains deposits was entertained by Owen, King, 

 Hayden, Leidy, Cope, Marsh, Scott, and Darton; it reached its cli- 

 max in King's proposal to give names to each of the great successive 

 lakes, beginning with those in the Mountain Region. This theory of 

 lake basins of very large extent on the Great Plains has been aban- 

 doned in the light of more exact paleontologic and geologic study. 



Among the geologists, Johnson, ^ Gilbert,"^ Haworth, and especially 

 Davis, who reviewed the whole subject in a broad and critical way, 

 have advocated a fluviatile and flood-plain origin. Hatcher, Fraas, 

 and recently Darton have also set forth strong reasons for fluviatile 



a Preliminary report on the geology and underground- water resources of the central Great Plains: 

 Prof. Paper U. S. Geol. Survey No. 32, 1905, pi. 35. 



b The history of opinion is fully traced in Davis, W. M., The fresh-water Tertiary formations of the 

 Rocky Mountain region: Proc. Am. Acad. Arts and Sci., vol. 35, No. 17, March, 1900, pp. 346-373. 



c Johnson, W. D., The High Plains and their utilization: Twenty-first Ann. Rept. U. S. Geol. Survey 

 pt. 4, 1901, pp. 601-741; Twenty-second Ann. Rept., pt. 4, 1902, pp. 631-669. 



d Gilbert, G. K., The imderground waters of the Arkansas Valley in eastern Colorado: Seventeenth 

 Ann. Rept. U. S. Geol. Survey, pt. 2, pp. 553-601. 



