334 L. D. CAIKXES EROSIOX AND EQUIPLAN ATIOX IN ALASKA 



vantage. Xowhere has the writer found this better illustrated thau in 

 the district in which, during the past summer, lie was engaged in connec- 

 tion with his regular field work for the Canadian Geological Survey. 

 There in a single small area two contrasting types of topography are 

 exhibited — one youthful, the other in a mature to old-age condition — 

 and the differentiation is almost, if not wholly, due to the unequal powers 

 of resistance which the dominant classes of bedrock in the district have 

 displayed toward the various erosive activities to which they have been 

 subjected. 



The topography throughout the greater part of the area examined is 

 characterized by low, generally well rounded, irregularly distributed 

 hills and ridges, separated by wide flaring valleys; but included in this 

 area is a belt whicli exliibits a well preserved high plateau, incised by 

 numerous gorgelike depressions. The topography at the beginning of 

 the present cycle is believed to have been very similar everywhere 

 throughout the district, but now differs so widely in various localities, 

 due to differential erosion. 



Forces are also at work on the already nearly flat upland surface, 

 tending to make this still more even and plainlike in contour than at 

 present by a process here termed equiplanation. 



Area 



The area in which the writer was engaged during the past summer,^ 

 and which well illustrates the greater number of the points contained in 

 this paper, extends along the 141st meridian (the Yukon- Ala ska inter- 

 national boundary) between latitudes 66° 08' and 6i° 00', or from the 

 Orange fork of Black Eiver northward to within about 30 miles of 

 Porcupine Eiver (figure 1). 



Geological Formations * 



With the exception of the Quaternary superficial deposits and a few 

 isolated occurrences of igneous intrusive rocks, which, however, are too 

 limited in extent to have had any perceptible influence on the general 

 topography of the district, the main geological formations are mentioned 

 in the following table : 



3 D. D, Cairnes : "A portion of the Yukon-Alaska boundary between Porcupine and 

 Yukon rivers." Sum. Kep. Geol. Surv., Department of Mines, Canada, 1911. 



* For more detailed descriptions concerning the .geological formations of this district 

 than the one contained in this paper, see 



D. D. Cairnes: "A portion of the Yukon- Alaska boundary between Porcupine and 

 Yukon rivers." Sum. Rep. Geol. Surv. branch, Department of Mines, Canada, 1911 (In 

 preparation). 



