128 A. C. SPENCER — PACIFIC MOUNTAIN SYSTEM 



This line of reasoning forms the most satisfactory basis for correlating 

 the peneplains of the various regions which have been described. 



Origin of the Pacific Mountains 



The latest dynamic revolution to the existence of which we have any 

 clue in the region bordering the Pacific ocean northward and westward 

 from the state of Washington is one which seems to have closed the 

 Mesozoic. Concerning this disturbance little evidence is now at hand, 

 and there are no means for determining the degree to which it affected 

 the structure of the region in question.^ However, at man}^ places in 

 the Pacific mountains there is abundant evidence of revolutions which 

 followed the deposition of the sedimentary rocks of the earlier Creta- 

 ceous and of the Triassic. Wherever observed, the rocks belonging to 

 these periods have been folded to a greater or less degree, and in many 

 cases, especially in the older strata, there has been an excessive amount 

 of disturbance. It is certain that these movements were of an erogenic 

 nature, and each in turn must have impressed a new physiognomy through- 

 out the regions which were affected ; but, whatever may have been the 

 respective reliefs resulting from folding and uplift, the events of subse- 

 quent periods have caused their complete effacement as controlling ele- 

 ments in topograph}'. The mountains which now exist are not belts of 

 high relief because they were raised during the disturbances which ensued 

 during the middle and later Mesozoic, or because of later adjustments 

 of the earth's crust amounting to geologic revolutions. The evidence 

 which has been given is believed to be sufficient to show that planation 

 of early Tertiary time reduced a very large area in British Columbia, 

 Yukon, and Alaska to the condition of a peneplain ; and since the com- 

 pletion of this denudation cycle all important earth movements within 

 the province have been restricted to continental uplifts unaccompanied 

 by tangential compression. While the nature of the later Tertiary and 

 recent uplifts is regarded as epeirogenic rather than orogenic, it is recog- 

 nized that tectonic mountains have been formed, but their elevation is 

 considered to have been entirely incidental to the broad regional move- 

 ments which have affected Alaska and the adjacent portions of North 

 America. 



The upraising of so vast a tract as the portion of North America here 

 considered to an average height of several thousand feet can hardly be 



*Dr G, M. Dawson is authority for the statement that where the Cretaceous rocks occur on the 

 seaward side of the coastal ranges they are found to have participated to a considerable degree 

 in an upturning which uflected the older strata as well. On the inner side of Vancouver island 

 some of the Mesozoic roeUs are reported to be probably as young as the typical Laramie. Am. 

 Jour, Sci. (.3d series), vol. xiiii, p. 435. 



