PRE-ORDOVICIAN ANB ORDOVICIAX 271 



scliists, and crystalline limestones, witli many intnisives, and is known to 

 he older than the Ordovician. The altered igneous rocks occurring in the 

 inetamorphic complex of Yukon can he conveniently grouped together 

 under the name "Pelly gneisses." They appear to he in part derived 

 from granitic, in part from rhyolitic rocks. There is some evidence of 

 their having been, in part at least, intruded as late as the Silurian. 

 Massive granites, in part as late as the Cretaceous, occur widely distrii> 

 uted as stocks and dikes. 



ORDOVICIAN 



The only Ordovician fossils which have been found in this province 

 are those described, by Mr Kindle (see pages 323-324) as occurring in a 

 l)]ue-gray limestone which outcrops on the Porcupine river near the Inter- 

 national boundary and has a thickness of about 600 feet (see page 323). 

 It is possible that some of the crystalline limestones now regarded as 

 forming an integral part of the pre-Ordovician schists may represent 

 infolded rocks of a later age, and the Ordovician may be represented by 

 some of these. Of the adjacent provinces the Ordovician is known to be 

 extensively developed in the Seward peninsula** alone, where it is repre- 

 sented by at least a part of the Port Clarence*' limestone, with a thickness 

 of probably several thousand feet. Some carbonaceous shales and lime- 

 stones occur in the upper Kuskokwim valley which carry graptolites, 

 referred by Charles Schuchert to the Ordovician.*® 



Dawson*" has described a black shale formation having a thickness of 

 some 1,500 feet, which is exposed on the lower Dease river near latitude 

 60° and which carries graptolites assigned by Lapham to the middle 

 Ordovician. 



These occurrences of deep-water sediments at such widely separated 

 localities as Dease river. Porcupine, Kuskokwim, and Seward peninsula 

 indicate the vast extent of the Ordovician sea in which they were laid 

 down. The relation of the Ordovician to the older terrains has not been 

 established, but is probably one of unconformity. There is some evidence 

 to indicate a profound metamorphism of the rocks here classed as pre- 

 Ordovician previous to the deposition of any of the known Paleozoic 

 terrains. On the other hand, however, in the Porcupine valley the only 



" Arthur .T. Collier, Frank L. Hess, T. S. Smith, and Alfred II. Brooks : The gold 

 placers of a part of the Seward peninsula. Bulletin no. .328, U. S. Geolosical Survey. 



*'• The Port Clarence may be in part Silurian. 



"Alfred II. Brooks and L. M. Prindle : An exploration in the Mount McKinley region. 

 Bulletin no. — , U. S. Geological Survey. (In preparation.) 



" George M. Dawson : Report on Yukon district and adjacent portions of British Co- 

 lumbia. Geological Survey of Canada. Annual Report, 1887, part B, pp. 94B-95B. 



XXVII— Bull. Geol. Soc. Am., Vol. 19, 1907 



