124 A. C. SPENCER PACIFIC MOUNTAIN SYSTEM 



and Chilkoot passes, where the summit plateau merges with the plateau 

 of the interior.* Again, as pointed out by Hayes, Tyrrell, and Brooks, 

 the Coast range diminishes in height northwestward from the vicinity 

 of L3^nn canal until its flat summit merges with the Yukon plateau. 

 Thus we have observations in two localities tending to show the direct 

 equivalence of the plateau of the interior with that of the Coast range 

 in its northern portion ; and since the continuance of both features 

 toward the south is a known fact, it is held that the two are equivalent 

 peneplains throughout the length of the Coast range. 



Westward from the region which has just been described, it is believed 

 that the Chilcat peneplain in the southeastern portion of the Saint Elias 

 range corresponds as an erosion feature with the adjacent Yukon plateau, 

 which stands at a considerably lower altitude. The contiguous and 

 overlapping portions of the Coast range and the Chilcat mountains are 

 both known to be dissected plateaus, and while, as has been noted, the 

 summit surface of the first descends toward the northwest and that of 

 the second is known to rise in that direction, the recent topographic 

 work of the United States Geological Survey shows that they have 

 similar elevations in the vicinity of Lynn canal, where they are separated 

 only by Chilcat inlet and the valley of which it is an eastward continu- 

 ation. On this account it seems more likely that the peneplains of the 

 two ranges are of the same age than that they were produced at different 

 dates. Considering together the two relations which have been men- 

 tioned, it may be accepted as most probable that the peneplain shown 

 in the summits of the southern portion of the Saint Elias range is the 

 physiographic equivalent of the Coast Range peneplain and of the Yukon 

 plateau. 



The correlation of the Chugach peneplain with the features whose 

 interrelations have been thus far discussed is more difficult because of 

 the great distance intervening between areas which can be directly com- 

 pared. However, the equivalence of the Chugach and Chilcat peneplains 

 is suggested by their mutual attitudes to the Saint Elias group of moun- 

 tains. It has already been stated that the general altitude of the plateau 

 summit of the Saint Elias range increases northwestward from the Chilcat 

 mountains toward the culminating peaks of the Saint Elias group. The 

 Chugach plateau also ascends from the vicinity of the Copper river 

 toward the same high summits, which, as viewed from the foothills of 

 mount Blackburn, in the Wrangell group, ma}^ be seen to rise high 

 above the general level of the country which surrounds them. As a 

 matter of fact, the region intervening between the Copper river and 



*C. W. Hayes : Expedition throngli the Yukon district, p. 129. 



