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



and shales of the Nation Eiver formation, generally considered to be of 

 Pennsylvanian age, succeed the Carboniferous shale series. More recent 

 tlian all these consolidated rook formations are the superficial deposits 

 of Eecent and Pleistocene times, which constitute a mantle obscuring the 

 older geological terranes, more or less, throughout the entire district. 



On the accompanying table of formations three typical sections are 

 shown, which are characteristic of the geological succession, so far as this 

 is known, in the northern, intermediate, and southern portions of this belt 

 along the 141st meridian. Since, however, the stratigraphy changes much 

 more rapidly toward the south than farther north, the northern section 

 applies to considerably the greater part of the belt, the intermediate and 

 southern sections referring only to the southern portion of the area. Also, 

 although the stratigraphy of the Paleozoic era varies so greatly in that 

 district, there are all gradations between the sections, as illustrated in the 

 table, these having been selected as being t}^ical of extreme phases. The 

 rapid changes in stratigraphy, particularly to the south of Harrington 

 Creek, are thought to be largely accounted for by the fact that the 141st 

 meridian, along which the sections were measured, lies practically at right 

 angles to the general trend of the geological structures and developments 

 of the region. 



METAMORPHIC ROCKS 



Pre-Camhrian ( ?) — Yul-on group.— -The members of the Yukon group 

 are exposed along the northern side of Yukon River, throughout an area 

 which within the belt mapped along the 111st meridian has an extent of 

 Irom 3 to 5 square miles. This constitutes the only known occurrence 

 of these rocks exposed along the Yukon-Alaska boundary line between 

 Porcupine and Yukon rivers, but this exposure constitutes a peripheral 

 portion of an extensive development of these rocks to the south of the 

 river. 



The members of this group are dominantly schistose in structure, and 

 within this particular area consist mainly of quartzite schists, schistose 

 amphibolites, and mica schists. All these rocks are much folded, faulted, 

 and distorted, and are so metamorphosed that in places it is difficult oi 

 impossible to determine their origin or original characters. 



Included in these schistose rocks there occur occasional beds or small, 

 irregular masses of limestone, which in places appear to represent in- 

 folded portions of more recent beds which formerly overlay the older rocks. 

 At one point at least on the south side of Yukon Eiver such proved to be 

 the case, as there the limestone was found to contain an abundance of 

 Paleozoic crinoid stems. In other places, however, the limestone is inter- 



