THE EOCENE PERIOD 



779 



zoic and Mesozoic; but this conclusion may be questioned. On 

 the basis of thickness, the showing of the system is great, as the 

 formations of Puget Sound, Coos Bay, Ore., and southern California 

 show. In -the western interior, too, the thickness of the system is 

 great, if the thicknesses of the beds deposited in the several succes- 

 sive areas of deposition, are added. Furthermore, any just estimate 

 of the duration of the period must take account of the great erosion 

 after the post-Laramie deformation before the deposition of the 



Fig. 526. Section showing the structure of the Eocene in western Oregon. 

 Eb, Eocene basalt; Ep (Pulaski formation), and EC (Coaledo formation), 

 Eocene. Length of section about 20 miles. (Diller, Coos Bay, Ore., 

 folio, U. S. Geol. Surv.) 



Fig. 527. Section a little south of the last, showing the relation of the 

 Eocene (Ep, Pulaski formation) to the Cretaceous (Km, Myrtle forma- 

 tion), as, amphibolite schist, and Ps, Quaternary marine sand. (Coos 

 Bay folio, U. S. Geol. Surv.) 



recognized Eocene began. On the physical side, therefore, there 

 is no warrant for the conclusion that the period was short. The 

 faunal developments of the period, too, were such as to make 

 great demands upon time. On the whole, it does not seem im- 

 probable that the period was as long as the average of those since 

 the beginning of the Paleozoic. 



The conditions requisite for so great thicknesses of terrestrial 

 sediment as occur in the Eocene of western North America are not 

 easily conceived, if the thicknesses are really as great as they have 

 been thought to be. If the areas concerned were in process of more 

 or less continuous warping, the depressions going down as sur- 

 rounding lands went up, or if troughs or basins of deposition were 

 produced by faulting, the bottoms sinking while their surroundings 

 rose, the conditions for thick sediments would perhaps be met. 

 It has sometimes been urged that these and similar formations are 



