312 BROOKS AXD KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



cussed, but attention will be directed to some of the salient features of 

 the geologic history. 



It has been shown that the oldest rocks are included in a complex series 

 of metamorphic sediments and intrusives. Among these sediments tine- 

 grained clastic material dominates, suggesting deposition at some dis- 

 tance from land. To what geologic age the sea in which these sediments 

 were deposited belongs is not Icnown. 



Seasons have been advanced in the foregoing pages for believing that 

 these metamorphic rocks belong to the pre-Ordovician. In view of the 

 great thickness of Cambrian strata (40,000 feet) reported b}' Dawson^^® 

 in the adjacent province of British Columbia, it seems fair to assume that 

 these sediments may, in part at least, be of Cambrian age. In still 

 greater doubt is the age of the metamorphism which converted these 

 sediments into schists and crystalline limestones. There were undoubt- 

 edly several periods of crustal movements during early or pre-Paleozoic 

 times; the latest producing any considerable amount of metamorphism 

 probably took place in late Silurian or early Devonian time. The evi- 

 dence of this lies in the probable Silurian age of the intrusive gneissoid 

 granites which were involved in this folding and also in the fact that 

 there appears to be a sharp line of demarcation between the Middle Devo- 

 nian and pre-Middle Devonian rocks. The semicrystalline character of 

 the oldest metamorphic rocks may be due, as suggested above, to their 

 having suifered alteration previous to the deposition of the known Paleo- 

 zoics, but also may be largely due to their having been more deeply 

 buried or lying closer to the axis of maximum disturbance. 



If any conclusions are warranted, on the fragmentary evidence at hand, 

 as to the extension of the Ordovician sea, it will l^e that it covered north- 

 em British Columbia, much of the Yukon and Kuskolrwim basins, and the 

 Seward peninsula. Indeed, everywhere that Ordovician sediments have 

 been found the usual absence of detrital material would indicate a much 

 wider extent of the sea. As the Ordovician fauna of Alaska is European 

 and Asiatic rather than North American, a barrier is to be expected be- 

 tween this sea and the other Ordovician seas of ISTorth America. 



There is no evidence of a crustal movement intervening between the 

 Ordovician and Silurian of Alaska. If, however, the fragmental rocks 

 of the Eampart region prove to be Silurian, as believed, there must have 

 been a very decided change in the physical conditions, suggesting a crustal 

 disturbance of some magnitude. It has been shown that the Silurian 



"" George M. Dawson : Geological record of the Rocky Mountain region In Canada. 

 Bulletin of the Geological Society of America, vol. 12, 1901, p. 62. 



