THE EOCENE PERIOD. 213 



the thickness of the system should include the thicknesses of the beds 

 deposited in the several successive areas of deposition. King esti- 

 mated the maximum thickness of the Eocene near the 40th parallel at 

 10,000 feet. 1 Furthermore, any just estimate of the duration of the 

 period must take account of the great erosion after the post-Laramie 

 deformation, and before the recognized Eocene deposition began, in the 

 places where the beds are now known, for it is to be remembered that 

 the Eocene beds are generally unconformable on the Cretaceous. Thus 

 in western Oregon, the Cretaceous formations had been largely removed, 

 and the surface well advanced toward base-level after the post-Cre- 

 taceous deformation, before the incursion of the Eocene sea permitted 

 marine sedimentation within the present land area. After Eocene 

 sedimentation began, there was still time before the end of the period 

 for the deposition of 10,000 feet (as sedimentary beds are measured) 

 before the close of the period. We must not conclude therefore that 

 the Eocene period was short, because the system is thin in many parts 

 cf the continent. 



The conditions requisite for so great thicknesses of terrestrial sedi- 

 ment 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 region of sedimentation was in process of 

 more or less continuous warping, the depressions deepening as the 

 surrounding lands were elevated, or if troughs or basins of deposi- 

 tion were produced by faulting, the bottoms sinking while their sur- 

 roundings rose, the conditions for thick sediments would be met. It 

 has sometimes been urged that such formations as those of the Eocene 

 of the west are too thick to be subaerial, but it is not apparent that 

 it is more difficult to account for thick subaerial sediments, under the 

 conditions indicated, than to account for thick lacustrine or even 

 marine formations. 



The relations of the Eocene beds accumulated in lakes or on the 

 land are such as to indicate that both the attitude and the altitude 

 of the surfaces in the western half of the continent were very different 

 from those which now exist. The western part of the continent must 

 have been, on the whole, much lower than now, and, locally and 

 temporarily at least, without well-established drainage. The present 



1 King, op. cit., p. 541. 



