OF WESTERN NORTH AMERICA. 



25 



vations (Barbour, Darton, and others) as to the volcanic-dust com- 

 position of certain OHgocene to PHocene sands in Nebraska, Montana, 

 and Colorado. Following Merriam's" determination (1901) of the 

 volcanic-ash nature of the deposits of the John Day basin, the next 

 important step of recent years in relation to the Eocene lake basins 

 is the recognition by Sinclair^ (1906) that the deposits of the Bridger 

 basin, previously described as sandstones and clays, are also chiefly 

 of volcanic nature or tuffs. The neighboring Washakie basin deposits 

 are of ash (Sinclair, 1907). On preliminary examination the same 

 observer finds tuffs in the Torrejon, Wasatch, Wind River, and Uinta, 

 as well as in the Bridger; in other words, in the entire Eocene series. 

 The lower part of the Wind River formation, however, and probably 

 parts, of other basin deposits, appear to be true sandstones and clays. 

 Veatch^ (1906) confirms King's observation that just below the 

 typical CorypJiodon zone of Evanston are extensive "white beds" 

 largely composed of volcanic ash, which he names the Fowkes 

 formation. 



7. The manner of deposition of volcanic ash in these various 

 basins, whether blown about on a dry surface, in flood plains, or in 

 either extensive or shallow lakes, has not been fully determined. In 

 the Bridger formation the ash shows little evidence of prolonged 

 water erosion. Merriam rejects the lacustrine theory of the origin 

 of the John Day formation and speaks of showers of ash, with tuff 

 deposits on a plain occupied in part by shallow lakes." 



8. Admirable studies of the John Day Oligocene, in most of its 

 biotic and geologic aspects, have been made by Merriam,^ Sinclair,^ 

 and Knowlton.^ (See p. 67.) 



9. The Bridger formation, 1,800 feet in thickness, is the only Eocene 

 deposit which has been exactly examined^ from the standpoint of 

 geology, petrography, and paleontology. Wind-blown volcanic ash, 

 glass, and eruptive feldspar are large ingredients of this formation, 

 which contains no erosion materials from the adjacent Uinta Moun- 

 tains, such as we should expect to find. There is evidence of the 

 direct deposition of the ash in water, with some working over of the 



a Merriam, J. C, A contribution to the geology of the John Day basin: Bull. Dept. Geology, Univ. 

 California, vol. 2, No. 9, April, 1901, pp. 269-314. 



b Sinclair, W. J., Volcanic ash in the Bridger beds of Wyoming: Bull. Am. Mus. Nat. Hist., vol. 22, 

 1906, pp. 273-280. 



c Veatch, A. C, Geography and geology of a portion of southwestern Wyoming, with special reference 

 to coal and oil: Prof Paper U S. Geol. Survey No. 56, 1907. 



d Sinclair, W. J., New or imperfectly known rodents and ungulates from the John Day series: Bull. 

 Univ. California, Dept. Geology, vol 4, 1905, pp. 125-143. 



e Knowlton, F. H., Fossil flora of the John Day basin, Oregon; Bull. U S. Geol. Survey No. 204, 

 1902, 153 pp. 



/ The writer planned this survey in preparation for the United States Geological Survey monograph 

 on the titanotheres, desiring to ascertain whether or not the Eocene titanotheres were horizontally 

 distributed, i. e., in vertically successive life zones. As conducted by and reported on by Messrs Mat- 

 thew. Granger and Sinclair, partly for the United States Geological Survey, but chiefly for the Ameri- 

 can Museum expeditions, it succeeded far beyond our most sanguine anticipations. 



