92 



THE AGE OF MAMMALS 



whether washed down from the mountain sides, blown about on dry sur- 

 faces, distributed in flood plains or in extensive or shallow lakes, has not been 

 fully determined. Merriam ^ rejects the theory of the lacustrine origin of the 

 John Day Formation and speaks of "showers of ash with tuff deposits on a 

 plain occupied in part by shallow lakes." Undoubtedly the wind or atmos- 

 pheric currents were a great agent of distribution. As observed by Russell ^ 

 one wind-distributed deposit in Alaska occupies 52,280 square miles, and is 

 100 feet thick in places. Other deposits cover great areas not only in the 



Fig. 24. 



Mt. Poloe and volcanic cloud distributing volcanic ash. After a iiaintiug by 

 Charles R. Knight under the direction of E. O. Hovey. 



mountain but in the plains region. As shown in the accompanying picture 

 of Mt. Pelee, the dust is carried high up into the air by the explosive dis- 

 charges of steam and gas, and may be distributed over vast areas by the 

 wind. It is white unless adulterated with other substances, and resembles 

 powdered pumice. It consists of angular flakes of glass, generally too small 

 to be distinguished by the unaided eye. Its chief component is silica. 



The total or combined thickness of these Eocene depositions is enormous, 

 but since the deposits were partly contemporaneous and partly successive, as 

 shown by a careful study of the life zones which they contain (diagram on 

 p. 49), the net total Eocene deposition, deducting the thickness of over- 

 lapping deposits, was about 7,200 feet. 



4. Pacific coast, close of the Cretaceous. — The subsidence and erosion of 

 the western part of the western continent had almost established a connec- 

 tion between the Pacific gulf in California and Oregon and the old Mississip- 

 pian Sea of the Mississippi valley.^ The intervening isthmus not covered 



' Merriam, J. C, op. cit, 1901. 



^ Russell, I. C, Volcanoes of North America, New York, 1897, p. 286 fol. 

 'Smith, J. P., Salient Events in the Geologic History of California. Science, n.s., 

 XXX, no. 767, 1909, pp. 346-351. 



Vol. 



