TERTIARY FAUNAS OF THE PACIFIC COAST 233 



freshwater or swamp conditions. The regions thus affected include 

 a large part if not all of the Puget Sound and western Oregon provinces 

 and a considerable part of central California. How far these condi- 

 tions extended eastward into central Washington and Oregon it is not 

 possible to state owing to the covering of the Eocene by later volcanic 

 flows. It is quite possible, however, that certain portions of the 

 Sound country was at no time submerged under salt water, or if at 

 all only for very short periods, for Willis states' that coal occurs 

 both in the basal and upper portions of the Puget formation, which is 

 believed to cover the period from the Eocene into the Miocene. He 

 states further that "the physical history which is recorded in the 

 Puget formation is one of persistent but frequently interrupted sub- 

 sidence" in which ''the alternation of coal beds with deposits of 

 fine shale and coarse sandstone indicates that during this great sub- 

 sidence the depth of water frequently changed." He infers "that at 

 times the subsidence proceeded more rapidly, and that the deepened 

 water was then filled with sediment, until the tide-swept flats became 

 marshes, and for a time vegetation flourished vigorously in the 

 moist lowlands," this rotation being repeated intermittently. This 

 description of conditions is believed also to apply to much of Alaska, 

 western Oregon, and portions of the interior valley of central Cali- 

 fornia during the later Eocene. The epicontinental Eocene seas 

 were for the most part rather shallow and in the later Eocene particu- 

 larly were bordered by wide tide flats and marshes. 



In the region of Lower Lake in Lake County, Cal., in the Mojave 

 Desert immediately north of the Sierra Madre, and in the vicinity of 

 San Diego, the early Eocene (Martinez) sea was present, but later 

 receded and these particular areas are believed to have been dry land 

 during the later Eocene. The Mojave Desert basin may have been 

 covered with freshwater at this later period as lake deposits believed 

 to be largely of Eocene age are known from the region contiguous to 

 it. This would be in accordance with the conditions prevailing in 

 eastern Oregon^ and Washington^ where great lakes existed during 

 Eocene time immediately east of what is now the Cascade Range, 



1 Tacoma Folio, p. 2. 



2 J. C. Merriam, Bull. Dept. Geol. Univ. of Cal., Vol. II, No. 9, p. 286, 1901. 



3 G. O. Smith, Mount Stewart and Ellensburg Folios, Washington. 



