302 



University of California. 



I Vol. 2 . 



some of the same objections that have been made to the aeolian 

 theory. 



A large part of the John Day seems to have been deposited 

 under conditions somewhat different from any that have been gen- 

 erally recognized as governing the accumulation of extensive forma- 

 tions. Other somewhat similar deposits on this side of the con- 

 tinent appear to have been formed in much the same way. 



Having originally held to the lacustrine theory of origin for the 

 whole series, it would seem to the writer, after consideration in 

 lights most favorable to this theory of all the evidence obtainable, 

 that, as commonly accepted, it fails to meet fully the necessities of 

 the case for a large part of the section. If it is retained for these 

 beds it will be in some modified form. 



The writer would suggest that showers of ash, with tuff deposits, 

 on a plain, occupied perhaps in part by shallow lakes, like the 

 so-called lake-beds of the prairie region of the central plains, might 

 form accumulations similar to those in a large part of the series. 

 If the basin were kept close to base level, irregularities of the sur- 

 face would hardly be noticeable. Rapid deposition of ash or tuff 

 would occasionally form homogeneous beds of considerable thick- 

 ness, and would, perhaps, make the region temporarily uninhabit- 

 able for most forms of animal or plant life. During the intervals 

 between such periods of rapid deposition, the surface might be 

 partially covered with vegetation and become the home of such 

 animal forms as have left their remains in the fossil beds. The 

 deposition of small quantities of ash during these periods would 

 not necessarily be a hindrance to the occupation of the region by 

 animals or plants, more than the deposition of loess-like dust in 

 the central plains region has prevented its habitation by a complex 

 fauna and flora. Future investigations may show that some of the 

 very thin-bedded portions of the ash deposits were laid down in 

 water, and that the less distinctly stratified beds are largely ^fiolian. 



Fossil Remains. — The mammalian and molluscan faunas of the 

 John Day are already fairly well known from the numerous con- 

 tributions to the palaeontology of this region by Marsh, Cope, 

 Scott, Wortman, Stearns, and others, and the objects of this paper 

 do not require a further consideration of this subject. 



