DIFFEKEXTIAL EKUSION 339 



to have occurred in late Miocene, Pliocene, or even early Pleistocene 

 time. 



The present discussion, however, is not particularly concerned with 

 the origin of this plateau province. The purpose of the writer is to 

 show that an elevated surface having only slight relief at one time ex- 

 tended throughout the tract now known as the Yukon plateau, and that 

 the long, even, nearly horizontal summits, typical of many of the ridges, 

 and the multitude of mountain summits, which in many localities are so 

 characteristically flat-topped, and which everywhere rise to so uniform an 

 elevation, all represent remnants of this former upland. Concerning 

 these points, there is an abundance of positive, confirmatory evidence in 

 almost any locality in the region under consideration, as has been noted 

 by the various geologists who have studied this topographic province. 



In the portions of the area along the 141st meridian here being 

 considered, in which the Ordovician-Silurian limestones and dolomites 

 constitute the dominant bedrock formation, the upland is well preserved, 

 and considerable stretches of flat or but slightly undulating plateau occur 

 at an elevation of about 3,500 feet above sealevel. This elevated surface 

 truncates the various limestones and dolomite beds wherever these are 

 unconformable Avith the almost horizontal upland surface (plate 15). 

 The upland is dissected by numerous gorgelike depressions, which con- 

 stitute the present day drainage channels of the area; and these valleys 

 have characteristically, very abrupt walls, at the contact between which 

 and the upland surface, decided shoulders representing topographic un- 

 conformities, are everywhere in evidence. It is manifest, therefore, that 

 this upland was produced during a formei' topographic cycle, and conse- 

 quently, previous to the entrenching of the present valleys, an unbroken, 

 plainlike surface, well advanced in old age, must have extended over the 

 entire district. 



Outside the areas of limestone and dolomite beds, where the bedrock 

 consists largely of Mesozoic slates, phyllites, quartzites, etcetera, although 

 the plateau surface in many localities* is almost or quite destroyed (plate 

 16, figure 1), still, in numerous places, ridges occur witli remarkably 

 straight, nearly horizontal summits, which are practically all in align- 

 ment, and these, together with the prevailing mountain summits, present 

 a strikingly even skyline, showing the district to be a typical example of 

 the thoroughly dissected peneplanated upland (plate 16, figure 2). 



ERODINO A\D DISINTEGRATING PROCESSES 



In passing from the northern part of the district, where the bedrock is 

 composed prevailingly of limestones and dolomites, to the more southerly 



