THE EOCENE OF EUROPE AND NORTH AMERICA 



91 



account for the perfect preservation of the vertebrate remains which charac- 

 terize 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 descendants in this region and whose 

 only remains are the bony fragments found in these lacustrine deposits?" 

 Following Merriam's ' determination (1901) of the volcanic ash nature of the 



EOCENE DEPOSITS 



VOLCANIC ERUPTIVE AREAS 



IN 



THE SAME REGIOM 



Fig. 23. — [To left] Chief Eocene fossil-mammal-bearing Formations (larjicly volcanic 

 tuffs) in the Mountain Region of North America, and [to right] areas of \()lcanic rock (lavas) 

 in the same geographic region. 1. Puerco and Torrejon, N. Mex. 2. Wasatch, Utah and 

 Wyo. 3. Big Horn (Wasatch), Wyo. 4. Wind River, Wyo. 5. Huerfano, Col. 6. Bridgcr, 

 Wyo. 7. Washakie, Wyo. 8. Uinta, Utah and Col. 9. (?) Fort Union, Mont. 



Oligocene deposits of the John Day Basin, the next important step was the 

 recognition by Sinclair - (1906) that the deposits of the Bridger Basin (Middle 

 Eocene) were also chiefly of volcanic nature, or tuffs. The same author ' 

 in 1907 proved that the successive Washakie Basin deposits were likewise of 

 ash. This led to a thorough examination by the United States Geological 

 Survey through Albert Johannsen of samples of rock from the entire Eocene 

 Series, with the most interesting and conclusive results.^ 



The manner of deposition of the volcanic ash in these various basins, 



' Merriam, J. C, A Contribution to the Geology of the John Day Basin. Univ. Cal., 

 Bidl. Dept. GeoL, Vol. II, 1901, p. 26!). 



'Sinclair, W. J., Volcanic Ash in the Bridger Beds of Wyoming. Bull. Amer. Mas. Nat. 

 Hist., Vol. XXII, Art. xv, 1906, pp. 273-280. 



■'Sinclair, W. J., Science, n.s. Vol. XXVII, no. 685, Feb. 14, 1908, p. 254. 



■* Johannsen, .\lbert, in lilteris, 1909. 



