190 D. D. CAIRXES SECTION ALOXG YUKON-ALASKA BOUNDARY 



ish Columbia and the Western States, which, is officially considered by the 

 United States Geological Surrey to represent the latest Algonkian. As 

 the age of these Beltian rocks is still disputed, and as correlation between 

 formations so widely separated geographically is always open to criticism, 

 the writer has adopted the new term Tindir group for these rocks along 

 the Alaska- Yukon boundary, although lithologically they appear to be 

 strikingly similar to the Beltian rocks, and these latter are also thought 

 by most writers to occupy a position stratigraphically corresponding to 

 that assigned here to the Tindir group. 



Further, as the members of the Tindir group are but slightly meta- 

 morphosed, and as the rocks of the Yukon group are so highly metamor- 

 phosed, and since, also, as previously explained, the Yukon group is 

 thought to be almost undoubtedly older than the Tindir group, an uncon- 

 formity is supposed to exist between these formational groups. As before 

 mentioned, however, the writer did not observe the members of the Yukon 

 group in direct contact with rocks that could be positively identified as 

 belonging to the Tindir group. If the phyllites along Yukon River prove 

 to belong to the Tindir group, as the writer is inclined to think they do, 

 the supposed unconformity is est-ablished, as these phyllites rest uncon- 

 formably on the members of the Yukon group. 



It would thus now appear as very probable that the pre-Cambrian is 

 extensively developed in portions of Yukon Valley and elsewhere in Yukon 

 and Alaska, and that these rocks are divisible into an upper but slightly 

 metamorphosed division, the Tindir group, and a lower highly metamor- 

 phosed division, the Yukon group. The writer is quite well aware, how- 

 ever, of the difficulties that may possibly arise in attempting to use a 

 classification of this kind with the information at hand, and also realizes 

 the uncertainty attending any attempt to measure the ages of rock groups 

 by the mystic scale of metamorphism : still it is thought that this classifi- 

 cation, owing to its simplicity, may prove useful in case the pre-Cambrian 

 age of the Tindir and Yukon groups becomes established. It is realized, 

 however, that the problem concerning these older rocks is yet far from 

 being solved, and that a great amount of work still remains to be per- 

 formed before the intricacies of this extremely interesting problem will be 

 understood. 



Devono-Camtnan limestones and dolomites. — A series of limestone- 

 dolomite beds, including members of Cambrian, Ordovician, Silurian, and 

 Devonian age, are extensively developed along the 14:1st meridian between 

 Porcupine and Yukon rivers, and throughout the belt these rocks have 

 everywhere the same lithological characteristics, so much so that it is im- 

 possible in most places to determine, even approximately, the age of the 



