GEOLOGIC RECONNAISSANCE MAP OF ALASKA 699 



group of Triassic age, and on these the Kennicott formation rests uncon- 

 formably. The Tertiary in this district is represented by some small areas 

 of lignite-bearing Eocene sandstone, and by a great thickness of volcanics, the 

 latter merging with those of recent date. 



The succession in the Yukon region has not yet been well determined. It 

 appears that the so-called Birch Creek schists form the oldest sediments, and 

 these may rest on an older gneissic complex. Within the schistose series occur 

 beds of crystalline limestone. In some areas at least a massive limestone ap- 

 pears to form a higher member of the metamorphic series, but this is not 

 definitely established. A great thickness of greenstones, with which are inter- 

 calated some Middle Devonian limestones, form the next higher group, resting 

 unconformably on the older and more highly metamorphosed rocks. In some 

 parts of the basin a massive Carboniferous limestone forms the next higher 

 member of the succession. 



The Lower Cretaceous is represented by some calcareous sandstones and 

 black slates. As in southeastern Alaska, the upper Cretaceous and Eocene ap- 

 pear to be represented by an unbroken succession of sandstones and shales. A 

 formation made up of sands, clays, and gravels has been provisionally referred 

 to the Pliocene. 



In northern Alaska Schrader found a series of schists forming the basal 

 member of the succession, and this overlaid by a massive crystalline limestone. 

 The latter, on the evidence of a few obscure fossils, has been tentatively as- 

 signed to the Silurian. Both Devonian and Carboniferous beds have been 

 found in this region, but the stratigraphic succession is obscure. 



Lower Cretaceous rocks overlap the Paleozoics, both north and south of the 

 range, and on the Arctic slope are succeeded unconformably by Eocene beds. 

 These in turn are overlaid by Pliocene silts. 



Structure 



The parallelism between the bed-rock structures, the mountain ranges, and 

 the shoreline has been pointed out. In southeastern Alaska the dominant 

 structures trend northwest and then, near the one hundred and fifty-first merid- 

 ian, swing west and south. 



Three sections are presented to indicate some of the larger structural 

 features. The first reaches from Controller bay, through the Chugach and 

 Wrangell mountains, to the international boundary. On the coast of the section 

 are indicated the closely folded Tertiary beds, resting unconformably on the 

 metamorphic sediments which make up the Chugach mountains. These latter, 

 which are probably in part Paleozoic, are intensely deformed. They are 

 separated by a fault from the broad syncline which makes up the Wrangell 

 mountains. The basal beds in this syncline are Carboniferous, which are over- 

 laid unconformably by Mesozoic sediments, and these in turn are capped by 

 Tertiary and recent lavas. Another fault cuts off the northern area of the 

 syncline from a broad belt of closely folded Mesozoic sediments. North of the 

 Pleistocene silts, which floor the Tanana valley, the section traverses a belt 

 of schists with which are closely associated some gneissic rocks. A section 

 across the Alaskan range indicates a broad synclinorium of Mesozoic rocks 

 (chiefly Jurassic) resting unconformably on Devonian limestone on the west, 

 which in turn rests on phyllites and cherts, which have yielded some Ordovician 



