266 BKOOKS ANi) KINDLE PALEOZOIC EOCKS OF UPPER YtTKOli 



der-'' into several formations. These include mica, quartz mica, and 

 ampliibole schists, with some massive granite. Schrader provisionally 

 assigned these rocks to the Silurian, but, as has been shown elsewhere,^" 

 the evidence is equally strong for a pre-Silurian age. These schists strike 

 northeast into the unexplored region lying between Porcupine river and' 

 the Arctic ocean, and are believed to represent metamorphic phases of the 

 pre-Ordovician quartzites and slates of Porcupine river already referred 

 to. This correlation does not now admit of proof. 



The southern and larger area of the schistose rocks crosses the Yukon 

 above the International boundary, then extends northwest parallel to the 

 main valley (see maps, figures 1 and 2) to about the 146th meridian, 

 where it bends to the southwest and finally passes underneath the allu- 

 vium of the Tanana valley. On the north the boundary of these schists 

 is well defined, being marked by a belt of Paleozoic and younger rocks. 

 Less is known of the southern boundary, but it is in part defined by areas 

 of younger Paleozoic, Mesozoic, and Tertiary sediments. 



These metamorphic rocks are dominatingly made up of quartz and mica 

 schists, quartzites, and crystalline limestones. These limestones in some 

 places, as in the Portymile and White Eiver regions, occupy considerable 

 areas and in part form an integral member of the schists, but may be in 

 part yoimger terrains which have been infolded with them. In the latter 

 case they may be found to be equivalent in age to some of the Paleozoic 

 limestones to be described below. Intrusions are not uncommon in this 

 metamorphic series and are for the most part acid rock. Of these, gran- 

 ites are the most abundant and occur both in massive and gneissoid phases, 

 and in parts of the province occupy considerable belts. These granites, 

 gneissoid granites, and gneisses were long believed^^ to belong to the 

 Archean, but recent studies, notably those of McConnell and Prindle, 

 have shown that most of these are altered intrusives. 



This southern belt of metamorphic rocks was first described by Spurr,*' 

 who termed the quartzites and mica schist the Birch Creek series, and the 

 calcareous phases the Fortymile series. The earlier work of Dawson^'^ 



^ F. C. Schrader : Preliminary report on a reconnaissance along ttie Chandlar and 

 Koyubuk rivers. Twenty-first Annual Report, U. S. Geological Survey, part 2, pp. 471- 

 475. 



F. C. Schrader : Reconnaissance in northern Alaska. Professional paper no. 20, U. S. 

 Geological Survey, pp. 56-58. 



»<> Alfred H. Brooks : The geography and geology of Alaska. Professional paper no. 45, 

 U. S. Geological Survey, pp. 211-218. 



^ See "The Geography and Geology of Alaska." Professional paper no. 45, U. S. Geo- 

 logical Survey, pp. 210-212. 



32 rjijig geology of the Yukon gold district. Eighteenth Annual Report, U. S. Geological 

 Survey, part ill, pp. 140-156. 



^ G. M. Dawson : Report on an exploration of the Yukon district. Geological Survey 

 of Canada, no. 629. Ottawa, 1898. 



I 



