182 D. D. CAIRNES SECTION ALONG YUKON-ALASKA BOUNDARY 



Yukon Plateau province; and since this physiographic terrane in the 

 vicinity of the 141st meridian has a general westerly trend, it is cross-cut 

 by the meridian practically at right angles. Thus, in going from Xew 

 Eampart on the Porcupine, south to Yukon Elver, the line of travel is 

 transverse to the trend of the main topographic features of the district, 

 and consequently a considerable diversity of topography is encountered. 



In certain localities where the prevailing bedrock is limestone or dolo- 

 mite, the plateau characteristics are still well preserved, and extensive 

 tracts of upland occur having elevations of 3,000 feet or more above sea- 

 level. With the exception of these areas, the original plateau surface has 

 been almost or entirely destroyed^ and throughout the greater part of the 

 district the land surface has become thoroughly dissected. 



Two ranges or mountain groups are crossed by the boundary line, which 

 have summits rising to elevations exceeding 5,000 feet above sealevel, and 

 it is possible that one or both of these may be connected with the Eocky 

 Mountain system to the west, and thus constitute outlying lobes of that 

 physiographic terrane. It is more probable, however, that these are but 

 isolated mountainous areas included within the Yukon plateau. To the 

 north and south of these more rugged and mountainous areas, as well as 

 between them, the topography consists dominantly of well rounded, 

 irregularly distributed hills, and at frequent intervals throughout the dis- 

 trict westerly flowing streams are encountered which have in most places 

 deep, steep-walled valleys, the floors of which are as much as 5 miles in 

 width and from 900 to 1,200 feet in elevation above the sea. Nowhere 

 was any evidence of glaciation noted. 



General Geology 

 general statement 



The geological formations along the 141st meridian between Yukon and 

 Porcupine rivers are dominantly of sedimentary origin, but inclade some 

 intrusives and also a group of metamorphic rocks, the origin of some of 

 the members of which is uncertain. The sedimentary members range in 

 age from Eecent to Middle Cambrian, and include as well a group of 

 rocks^the Tindir group — the members of which are either Lower Cam- 

 brian or possibly pre-Cambrian in age; and not only are Mesozoic and 

 these possibly pre-Cambrian beds somewhat extensively developed, but in 

 addition each of the five systems of the Paleozoic from the Cambrian to 

 the Carboniferous is represented. Occasional dykes and small intrusive 

 bodies pierce the Mesozoic and Paleozoic beds in places, and greenstones 

 of various types have a considerable development in association with cer- 



