TERTIARY ROCKS, COLVILLE SERIES. 83 



The lower Colville contains the more indurated class of rocks and consists mainly 

 of partially consolidated silts in beds 6 to 8 feet in thickness. They are usually light 

 slate-colored or ash-colored, and constitute about one-half of the lower Colville 

 section, and are generally much less consolidated toward the top than near the base. 

 The harder rocks, which increase in volume toward the base of the section, include 

 impure, dull gray, medium to fine grained sandstone with detrital lignitic plant 

 remains; slate-colored and brownish calcareous shale, with disseminated undeter- 

 minable vegetable detritus; lignitic coal in layers 1 to 5 feet in thickness; dark 

 slate-colored or brownish chert, containing cavities incrusted with chalcedonic silica; 

 rusty brown, very ferruginous sandstone or impure iron stone, and some iron- 

 stained siliceous conglomerate, which also contains lignitic vegetable remains. 

 There are also a few layers of hardened silts, forming a rock of very tine texture, 

 resembling soft, smooth hone stone. 



The lower Colville is tentatively classed as Oligocene, on account of the presence 

 in it of lignite beds and vegetable remains, and from its resemblance to the Kenai 

 beds occurring elsewhere in Alaska, and also on the ground of its relation to the sup- 

 posed Pliocene silts which it immediately underlies. Several specimens of the shale 

 collected from it, bearing lignitic plant remains, were examined by Doctor Dall, 

 who reports that "the vegetable fragments are probably the needles of Sequoia 

 langsdm'ffii Heer. These beds containing vegetable remains in northern Alaska are 

 usually Upper Oligocene." 



Upper Colville {Pliocene). — This portion of the section is practically free from 

 indurated rock. It consists of nearly horizontally stratified beds of fine gray, slate- 

 colored, or ash-colored calcareous silts, containing faunal remains. It is tentatively 

 assigned to the Pliocene on the basis of fossils collected in place by the writer in the 

 bluff (near its top) on the west side of the Colville about a mile north of the seven- 

 tieth parallel (PI. XIV, A). These have been reported on by Doctor Dall as follows: 



Fossils from upper Colville series in bluff on west side of Colville River 1 mile north of seventieth parallel. 



Chrysodomus, 2 species, both undescribed and very interesting; the hrst Tertiary arctic shells 

 (not Quaternary) I have ever seen. Perhaps they are Pliocene. 

 Amarrropsis sp., fragments. 

 Tachyrhynchus polaris Beck. 

 Macoma frigida Hanley. 

 Macoma incongrua von Martens. 



Astarte semisulcata Leach (possibly Quaternary intrusion). 

 Saxicava arctica L. 



