THE EOCENE PERIOD. 



247 



developed by the wind may often be distinguished from that developed 

 by water, but it is not clear that the distinction can always be made 

 where exposures are limited (Fig. 437). 



In Colorado there was a small area of deposition in the South Park. 

 The beds (Florissant) deposited here consist largely of volcanic ash, and 

 are famous for the extraordinary number of insects which they contain. 



Some of the John Day beds of Oregon/ unconformable above the 



Fig. 437. — Section of stratified dune sand (recent). South end of Lake Michigan. 



(Bastin.) 



Eocene (Clarno), are probably to be referred to the Oligocene. This 

 area of aggradation occupied but a few hundred square miles, but in 

 it sediments accumulated to a thickness which has been estimated at 

 several thousand feet. They consist, in considerable part, of volcanic 

 ash and tuff. The youngest of the John Day beds seem to be younger 

 than the White River beds, and perhaps should be classed as Miocene. 

 The John Day beds, Oligocene and later, appear to be largely of eolian 

 origin, but the upper part of the series contains fresh-water shells. 2 



1 Dall, 19th Ann. Rept, IT. S. Geol. Surv.; Merriam, Jour. Geo]., Vol. IX, pp. 71-2, 

 and Bull. Univ. of Cal., Vol. II, p. 270 et seq. 



2 Merriam, Bull. Univ. of Cal, Vol. II, p. 270 et seq. 



