GEOLOGICAL FORMATIONS 337 



The quartzites range from nearly wliite to dark grey in color, and arc 

 typically massive, with a sugar-grained texture. Occasional beds, how- 

 ever, contain a certain amount of mica and chlorite, which, in places, are 

 arranged, as the result of metamorphism, along definite planes between 

 layers of purer quartzite, giving to the rocks a distinctly gncissoid habit. 



The sandstones and shales were only rarely noted, and are the less 

 metamorphosed equivalents of the slates, phyllites, and quartzites. 



The dolomites and magnesites almost invariably weather rough and- 

 red, due to the considerable amount of iron ore they contain. The dolo- 

 mite beds are, in places, as much as 200 feet or even more in thickness ; 

 but the magnesite beds rarely exceed 10 feet, and occasionally occur 

 interbanded with the slates and dolomites in layers less tlian 2 feet in 

 thickness. 



An accurate estimate of the aggregate thickness of this group of rocks 

 has not been made, since no place was found where the uppermost beds 

 are preserved, and only small portions of the section could be observed 

 at any one place. Moreover, on account of the metamorphosed condi- 

 tion of the rocks, it was difficult in most places to determine the dip and 

 strike of the beds. The group is, however, at least 6,000 feet in thick- 

 ness and may be considerably more. 



The only fossils obtained from these rocks are poorly preserved, and 

 were found within 100 feet of the underlying Carboniferous rocks ; of 

 these Dr. T. W. Stanton, of the United States Geological Survey, says : 

 "My judgment is that these fossils are not older than Mesozoic and they 

 may be Cretaceous, though there is no definitely distinctive Cretaceous 

 fossil among them, and they do not seem to fall into any fauna known 

 to be from that region." 



Tn this paper, which is chiefly concci-ned with physiography, th(^ Meso- 

 zoic and Ordovician-Silurian beds are mainly considered, as these arc 

 the dominant geological terranes, and have given rise to the two extreme 

 types of topography exhibited in the district. The Carboniferous i-ocks 

 are of comparatively small extent: consequently the linu^stones of this 

 series, unless specifically mentioned, are not intended to i)c included by 

 the term "limestones and dolomites," which is frequently emi)]oyed with 

 reference to the Ordovician-Silurian beds. 



Differential Erosion 



yukon plateau 



The main area under consideration lies within and toward the north- 

 ern edge of what is generally known as the Yukon Plateau physio- 



