24 



CEXOZOIC MAMMAL HORIZOXS 



GEOLOGIC HISTORY OF MOUNTAIN BASIN DEPOSITS 

 OF THE EOCENE AND OLIGOCENE. 



The combination by faunistic correlation of all the Eocene sections, 

 as represented in fig. 1, gives a total thickness of 7^200 feet. 



The deposits are distinguished by the following chief characters: 



1. The axes of the mountain ranges were the same as at present. 

 The mountain ranges, in relation to the surrounding country, were 

 probably higher than at present, because we must allow for two to 

 three million years of erosion. 



2. The Eocene drainage systems were broadh' the same as the 

 modem, namely, the s^^stems of Colorado River, Arkansas River, the 

 Bighorn branch of Missouri River, and Columbia River. In details, 

 however, the drainage systems have certamly been modified by uplift 

 and erosion. 



3. The deposits all He in the same great mountain basins or moun- 

 tain valleys in which they were originally deposited. (See PI. I.) 



4. Except close to the mountam foothills (e. g., Wasatch of the 

 Bighorn Basin) there has been comparatively little Eocene or post- 

 Eocene disturbance, because these deposits are still horizontal or at 

 gentle angles with their original horizontal position. 



5. The surrounding mountain ranges were interspersed with active 

 volcanic peaks; the upper Colorado River basin especially was sur- 

 rounded by a circle of volcanoes which poured out their lava and 

 widely distributed their ashes. 



6. From preliminary lithologic examinations the Eocene deposits 

 have been found to consist largely, sometimes exclusively, of vol- 

 canic-ash materials." The subject has an interesting histor}': In 

 1885-86 Merrill and Peale determined the volcanic-ash origin of the 

 '^Bozeman lake beds'' in Gallatm County, Mont. Peale's conclu- 

 sions were interestino^. ^ These observations are in line with King's^ 

 (1876) previous recognition of volcanic-ash strata in the t^^ical 

 Wasatch of Evanston, Wyo., immediately underlying the true Cory- 

 pJiodon zone of Marsh, with. Wortman's note as to the volcanic-ash 

 nature of the Huerfano basin Pliocene, and with, a number of obser- 



a It is interesting to note the similar volcanic-ash character of the Santa Cruz, the chief Miocene for- 

 mation of Patagonia. 

 b Peale, A. C, Science, vol. 8, Aug. 20, 1886, p. 163. The article concludes as follows: 

 * ' Will we not, therefore, have to cut down very materially the great length of time generally l^elieved 

 to have elapsed in this region from the beginning of this lacustrine period to the present time, when 

 we find that a great portion of the sediment that once filled the lakes is due, not to the products of 

 erosion, as has hitherto been supposed, but to repeated showers of volcanic dust? Again, do not these 

 volcanic materials, which must have fallen in showers over a large extent of country— accumulating 

 in some cases in beds 40 to 90 feet thick — account for the perfect preservation of the vertebrate remains 

 which characterize the formations in so many parts of the West; and is there not also suggested one 

 possible cause for the extinction of some of the many groups of animals which have at present no de- 

 scendants in this region, and whose only remains are the bony fragments found in these lacustrine 

 deposits? " 



cAm. Jour. Sci., 3d ser., vol. 11, 1876. pp. 478-480. 



