132 A.C.SPENCER — PACIFIC MOUNTAIN SYSTEM 



back to the Eocene. Consequent! 3- the present relief has originated since 

 the close of the first great division of the Tertiary period.^ 



The elevator}^ movement which has affected the portion of North 

 America here outlined has been differential in the extreme, but the 

 movements have been contemporaneous in different portions of the 

 region. If the conclusions which have been presented are warranted, 

 there need be no question of the relative dates of the different component 

 ranges of the northern part of the Pacific system, since all are of the 

 same date. The fact is recognized, though it has not been discussed 

 because of the complications of the larger problem which it would in- 

 troduce, that separate uplifts have taken place at different times, with 

 intervening periods of relative stability. There can be little doubt, also, 

 that movements are still in progress, since a portion of the Saint Elias 

 range is known to have been raised, in Pleistocene time, as observed by 

 Professor Russell. 



Summary 



The following propositions embody the principal suggestions whicli 

 have been presented in the foregoing pages : 



I. The interior and Yukon plateaus of British Columbia, Yukon, and 

 Alaska have been previously recognized as uplifted peneplains, and in 

 the Copper River region the summits of the high mountains have been 

 described as upraised baselevel surfaces. It now appears that the uni- 

 form summits which are found over the greater portion of the Pacific 

 Mountain system in the north are also representative of elevated pene- 

 ])lains which have suffered deep dissection. 



II. The peneplains of the different portions of the coastal mountains 

 and of the inland plateaus can be correlated one with another. The 

 antecedent nature of the rivers which cross the present coastwise barrier 

 demonstrates the identity of the ancient erosion surfaces throughout the 

 region outlined. 



III. The Pacific province was raised after the production of the 

 peneplain by erosion extending through Eocene time, mainly through 

 uplifts of a continental character. Regional elevation was accom[)anied 

 by warping, flexure, or displacement, raising tectonic blocks which have 

 not been effaced by subsequent erosion ; but there has been no moun- 

 tain-building due to tangential compression. 



♦Author's note.— While this paper was in the printer's hands I learned the results, soon to be 

 published, of the careful studies of Bailey Willis and George Otis Smith in the Cascade mountains 

 of central W^ashington. These observations show that the Cascade mountains were formed by 

 direct deformation since baseleveling in Pliocene time— a conclusion which is so closely in accord 

 with my ideas as to the origin of the Pacific mountains that I am led to expect future studies 

 to demonstrate that the beginning of the deformation which I have suggested may eventually be 

 closely correlated with the uplift of the Cascade range. The rugged character of the mountains 

 of the Pacific province would of itself suggest a very recent date for their uplift— a point which 

 might well have been emphasized in my discussion. 



