METAMORPHIC ROCKS 185 



calated somewhat regularly, and has every appearance of being and prob- 

 ably is contemporaneous with the schistose members. This limestone has 

 become completely altered into marble and is in most places much sheared 

 and brecciated. 



The name Yukon group^ was first employed by the writer to designate 

 these rocks along the 141st meridian/ and is intended to include all these 

 older metamorphosed, schistose, and gneissoid rocks both of sedimentary 

 and igneous origin. Similar schistose and gneissoid rocks are extensively 

 developed in portions of Yukon and Alaska, and practically all geological 

 workers who have recently studied them Have considered these rocks to 

 constitute the oldest geological terrane in the particular district investi- 

 gated. These rocks have thus been variously classed as pre-Devonian, 

 pre-Silurian, and pre-Ordovician, according to the age or supposed age 

 of the oldest overlying beds. In summing up the information concerning 

 these rocks in the Upper Yukon basin, Brooks and Kindle state :^^ "The 

 data at hand justify the statement that below the rocks of known age 

 (certainly older than Devonian and probably than the Ordovician) there 

 is a complex of metamorphic sediments and igneous rocks which is widely 

 distributed in the Upper Yukon basin." These writers class these rocks 

 as pre-Ordovician.^^ 



The small development of schistose rocks included in the belt along the 

 141st meridian, here being considered, constitutes a peripheral portion of 

 an extensive development of these older metamorphic rocks described by 

 the above mentioned authors, the extent of these rocks along the Upper 

 Yukon, Alaska, being indicated approximately on figures 1 and 2 in the 

 paper to which reference has just been made. These rocks are considered 

 by these writers to be pre-Ordovician, as the oldest fossil remains found 

 in the overlying limestones are of Ordovician age. The finding by the 

 writer of Middle Cambrian fossils in the corresponding limestones along 

 the 141st meridian thus throws additional light on the age of these meta- 

 morphic rocks. As is mentioned in describing the Tindir group, these 

 rocks underlie a thick series of limestone-dolomite beds, and Middle Cam- 

 brian fossils occur in the limestones some distance from the bottom of the 

 series. It would thus appear as possible that the Lower Cambrian is rep- 

 resented by the lowest beds of the limestone-dolomite series. In any case 

 the Tindir group is either Lower Cambrian or pre- Cambrian. There is 



8 It is realized that the term "Yukon silts" has been previously applied to the silts of 

 the Yukon basin, still it is not considered that this will conflict with the use of the term 

 "Yukon group" for the pre-Cambrian ( ?) metamorphic rocks of the North. 



» D. D. Cairnes : Along portions of the Yukon-Alaska International Boundary line be- 

 tween Yukon and Porcupine rivers. Geol. Surv. of Canada, Sum. Rept. for 1912. 



" A. H. Brooks and E. M. Kindle : Paleozoic and associated rocks of the Upper Yukon, 

 Alaska. Bull. Geol. Soc. Am., vol. 19, 1908, p. 270. 



" Ibid., pp. 264-271. 



