OUTLINE OF GEOLOGIC HISTORY. 



53 



Table of Geologic Formations. 



Sedimentary rocks. 



Pleistocene 



Tertiary (Colville series) .{ 



f Pliocene . 



Oligocene . 



llesozoic 



Paleozoic . 



Dpper Cretaceous (Nan- 

 ushuk series). 



Cretaceous (?) (Berg- 

 man series). 



Lower Cretaceous (Ko- 

 yukuk series). 



Lower Cretaceous (An- 

 aktuvuk series) . 



Jura - Cretaceous ( Cor- 

 win series). 



Lower Carboniferous 

 (Fickett series). 



Devonian (Lisburne for- 

 mation). 



Silurian (probably) 

 (Totsen series). 



Upper Silurian (Skajit 

 formation). 



Pre-Devonian (proba- 

 bly) (Stuver series). 



[Recent stream gravels, dune sands, mud flats, silts, 

 < beach gravels, Gubik sand, ground ice, and gla- 

 { cial drift. 



/Nearly horizontally stratified, fine-grained calcare- 

 \ ous silts. 

 Sand, clay, shale, soft sandstone and limestone 



with lignites, hard ferruginous sandstone, and 



conglomerate. 



[Soft sandstone, limestone, shale, and coal. 



[■Sandstone, slate, grit, conglomerate, and coal. 



\lmpure limestone, sandstone, slate, and associated 



I igneous rocks. 



1 Principally impure sandstone orarkose, with some 



J conglomerate or grit. 



\Sandstone, impure limestone, shale, and bitumi- 



/ nous coal. 



\Phyllites, slate, limestone, sandstone, quartzite, 



/ grit, and conglomerate. 



[•Limestone and shale. 

 [Mica-schists and quartz-mica-schists. 

 [■Schistose and micaceous limestone. 

 [Conglomerate, quartzite, slate, and shale. 



Igneous rocks. 



Post-Silurian dike (?) rocks, associated with the Skajit formation. 



Cretaceous and post-Cretaceous dike rocks, associated with the Koyukuk series. 



The oldest rocks encountered in the field consist of several metamorphic series. 

 They are principally of sedimentary origin, and have acquired their present character 

 largely by processes of mountain building, which, broadly speaking, seem to have 

 been in progress intermittently since Middle Paleozoic time, and are probably still 

 going on. 



These metamorphic rocks are exposed only in the mountainous portion of the 

 field. The different series are here designated by the names Skajit, Totsen, Stuver, 

 Lisburne, and Fickett. Though the Totsen series is mainly sedimentary, it includes 

 also some greenstone-schists of igneous origin, which appear to be old basaltic flows, 

 but may be intrusives. 



With probably a single exception, igneous rocks were not observed with any of 

 the other metamorphic series. This exception was on John River, near the middle 

 of the Skajit formation, at a point about midway between camps of June 23 and 

 June 24. Here, in the face of the upper part of a cliff which rises steeply to a height 

 of 2,000 feet above the edge of the stream, as seen from the opposite side of the 

 river, looking northeast, but which, owing to difficulty of access and lack of time 

 was not visited, the limestone is cut by what appears to be two classes of dikes (but 



