MBTAMORPHIC ROCKS 187 



also be included in the Yukon group, as well probably as the Greenstone 

 schists^^ described by these writers and others. Brooks has also used the 

 name Kotlo series^^ in a general way to include all the older sedimentary 

 rocks which he regards as of Lower Paleozoic or pre- Cambrian age, but 

 also adds that "this series is of especial interest, because it probably em- 

 braces all the gold-bearing rocks of the Upper Yukon.^^ McConnell has 

 shown, however, that the Klondike series are the principal gold-bearing 

 rocks of the Klondike district, and these rocks are dominantly of igneous 

 origin. From the descriptions of the Kotlo series, therefore, it seems some- 

 what doubtful whether or not only rocks of sedimentary origin are in- 

 tended to be included. Thus the name Kotlo series may be synonymous 

 with the term Yukon group, but it would appear as more probable that 

 the later name is the more comprehensive and includes the former. 



The members of the Yukon group occurring within the particular belt 

 along the 141st meridian, described in this paper, are, dominantly at 

 least, of sedimentary origin, and thus belong to the Nasina series. The 

 name Yukon group is used, however, to avoid chance of error in correla- 

 tion, as it is necessary to know the origin of the schistose member to be 

 able to apply the other f ormational names that have been employed. It is 

 also thought that the term Yukon group should be a useful field name, as 

 under this group may be included all older metamorphic, probably pre- 

 Cambrian, schistose and gneissoid rocks that are encountered, regardless 

 of their origin, which is often difficult or impossible to determine. 



The writer recognizes the difficulty that exists in dealing with these 

 older rocks, and also realizes that in many places there exists a great 

 amount of uncertainty concerning their age and origin. Still it seems 

 fairly certain that there does exist a pre-Cambrian metamorphic com- 

 plex underlying all the sedimentary rocks of known age in the Upper 

 Yukon Valley and other portions of Yukon and Alaska ; also these rocks, 

 which have previously been considered to be pre-Ordovician or younger, 

 would now appear to be all or at least dominantly of pre-Cambrian age. 



DOMINANTLY SEDIMENTARY ROCKS 



Lower Cambrian or pre-Camhrian — Tindir group. — The members of 

 the Tindir group within the particular area under consideration in this 

 paper are exposed mainly in two localities, namely, along Porcupine 

 River, in the northern end of the district, and between Ettrain and Har- 

 rington creeks, nearly 150 miles farther south. These rocks extend along 



« A. H. Brooks : Op. cit, p. 470. 



2» A. H. Brooks : A reconnaissance from Pyramid Harbor to Eagle City, Alaska. U. S. 

 Geological Survey, 21st Ann. Kept., part ii, 1899-1900, pp. 357-358. 

 » Ibid., p. 358. 



