272 BROOKS AiS^D KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



locality where the Ordovician has been found in contact with older rocks, 

 the latter had been little altered. There is, therefore, little clew to the 

 age of this first great period of diastrophism. 



SILURIAN 



Terrains which will probably be ultimately referred to the Silurian 

 find a wide distribution in central Alaska, but only a few of them have 

 yet yielded definite Silurian fossils. In the second part of this paper 

 Mr Kindle describes the Silurian of the Porcupine, embracing two dif- 

 ferent terrains — a lower, made up of about 2,500 feet of magnesian lime- 

 stone and black shale, and a higher, including about 25 feet of black 

 shale, l3dng immediately below and unconformable with Devonian sedi- 

 ments. 



A westerly extension of these Porcupine rocks would carry them into 

 the Koyulaik basin (see maj), figure 1), where Schrader has given the 

 name Skajit formation to some crystalline limestones and schists which 

 he provisionally assigned to the Silurian*^ on the evidence furnished by 

 an obscure fossil. It seems quite likely, however, that the Skajit may 

 belong with the metamorphic schist series. 



The section studied by the writers along the upper Yukon does not 

 include any rocks which have been definitely assigned to the Silurian. 

 There is, however, a group of heavy crystalline limestones, slates, and 

 quartzites of unknown age which may prove to be Silurian, but are here 

 included with the description of the Devonian. 



In the southwestern part of the province, especially in the region lying 

 between Fairbanks and Eampart, there is an extensive development of 

 rocks embracing cherts, slates, and feldspathic sandstones and quartzites, 

 with considerable greenstone and some limestone, which have been pro- 

 visionally assigned to the Silurian.*^ These are younger than and un- 

 conformable with the metamorphics and appear to be older than the 

 Devonian terrains of the same region. As, however, they are lithologic- 

 ally very similar to the Devonian and have yielded (Silurian) fossils at 

 only one locality, it is quite possible that a part of this series may be of 

 Devonian age. The Silurian fossils were found bv Prindle in a heavv 

 gray limestone on Quail creek about 12 miles northeast of Eampart. Mr 

 Kindle^" reported on these fossils as follows : 



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

 Geological Survey, pp. 56-58. 



" L. M. Prindle : The FairljanIvS and Rampart Quadrangles. Bulletin no. 337, U. S. 

 Geological Survey, pp. 18-20. 



^ L. II. Prindle : The Fairljanks and Rampart quadrangles. Bulletin no. 337, U. S. 

 Geological Survey, pp. 21-22. 



