GEOLOGIC RECONNAISSANCE MAP OF ALASKA 697 



Keeping this fact in mind, it will be possible to trace the stratigraphic subdi- 

 visions even on this very incomplete map. 



A belt of metamorphic rocks striking parallel to the Pacific coastline has 

 been traced northwestward through the panhandle and appears to find its ex- 

 tension in the Chugach mountains and in Prince William sound. In south- 

 eastern Alaska this belt includes various terranes, varying in age from Silurian 

 or older to the Permian, with possibly some Triassic. Some Cretaceous beds 

 are found infolded with it. It is cut off from the Paleozoic rocks of British 

 Columbia by the broad belt of intrusives which make up the Coast range. At 

 the westward extension of the belt Mesozoic beds overlap its inland margin. 

 These Mesozoic beds are continued to the southwest, forming the country rock 

 of the Alaska peninsula. 



A second belt of metamorphic sediments is traceable through inland Alaska. 

 This includes highly altered rocks, ranging from Silurian or older to Devonian. 

 This zone ends in the Kuskokwim valley, where a broad belt of Cretaceous sedi- 

 ments mantels the metamorphic terranes. This belt is broken by an area of 

 gneissoid rocks, but these, though first assigned to the Archean, are now be- 

 lieved to be largely altered intrusives. The metamorphic rocks appear again 

 in the Seward peninsula and in northern Alaska and here constitute a third 

 belt. i \ ■■■ W^ 



Little is known of the geology of the Rocky mountains of Alaska, except along 

 the one hundred and fifty-first meridian, where Schrader's studies have shown 

 them to be made up of closely folded Paleozoic terranes. 



A belt of Permian beds, made up of slates and limestones, has been identified 

 along the Seward margin of the Coast range and in the Copper River basin. 

 Devonian beds are widely distributed, but the largest areas occur in the 

 Yukon-Tanana region, where they are chiefly limestones and volcanics. 



The Mesozoic period is represented by the Jurassic and Triassic rocks of the 

 Copper River region, the Alaska range ; also by two broad belts of Cretaceous 

 rocks, one of which stretches northeastward from Bering sea to where it over- 

 laps on the Paleozoic terranes near the southern front of the Rocky mountains, 

 and the other stretches east and west across northern Alaska. The Tertiary 

 period is represented chiefly by Eocene beds, which occur in broken areas along 

 the seaward margins of the province. In the Yukon basin Eocene beds are 

 found far inland, close to the international boundary. These are probably of 

 lacustrine origin. 



Intrusive rocks, among which granitic types dominate, are very abundant in 

 southeastern Alaska. A broad belt of granitic rocks forms the backbone of the 

 Alaska peninsula, and smaller rocks occur in the mountains to the northeast. 

 All of these intrusives appear to be of Middle or Upper Jurassic age. The 

 smaller masses of granite, so abundant in the Kuskokwim valley and found in 

 the Seward peninsula, are probably of Tertiary age. 



Recent and Tertiary volcanic rocks are widely distributed, and in the Alaska 

 peninsula, Mount Wrangell region, and in the Bering sea littoral cover large 

 areas. 



The general stratigraphic succession in Alaska, so far as determined, is as 

 follows : Some gneisses and crystalline schists have been provisionally referred 

 to the basal member of the succession. These are succeeded by a great com- 

 plex of metamorphic sediments, intruded by many igneous rocks whose age 



