EARLIER TERTIARY (EOCENE AND OLIGOCENE). 771 



K-L. 10. COAST RANGE OP OREGON. 



According to Diller/^^ Eocene rocks form the mass of the Coast Range from 

 a point near the Columbia to Coquille River. Three groups of strata are distin- 

 guished — (1) the oldest, composed in general of igneous sediments closely related to 

 lavas of earlier or contemporaneous eruption; (2) shales, containing here and there 

 much material of igneous origin; and (3) sandstones, forming the uppermost group. 

 Diller describes many local occurrences and cites Ball's determinations of fossils 

 from specific places. 



These Eocene terranes constitute the Arago group (named from exposures near 

 Cape Arago), which is divided into the "Pulaski" and Coaledo formations.^''^ 



The " Pulaski" formation comprises all the Eocene strata below the coal-bearing 

 Coaledo formation in the Coos Bay and Port Orf ord quadrangles. It consists 

 chiefly of soft yellowish sandstone interstratified with thin beds of shale but includes 

 also small bodies of limestone composed largely of remains of algse and Foraminifera. 

 During the "Pulaski" epoch and particularly toward its close there were volcanic 

 eruptions which resulted in basalt flows and tuff beds. 



The Coaledo formation consists of sandstones and shales with coal beds. The 

 strata were deposited chiefly in brackish waters and thus contrast with the marine 

 "Pulaski." Sandstones predominate in the lower part of the Coaledo, whereas in 

 the upper portion light-colored shales are characteristic. 



The Arago group, composed of the "Pulaski" and- Coaledo formations, com- 

 prises about 10,000 feet of strata. The uppermost portion as measured near Cape 

 Arago consists of foraminiferal and other shales (2,200 feet) and sandstones (850 

 feet) . The sandstones are distinguished by Dall ^"^ as the Tunnel Point beds. 



Regarding the correlation of the Arago group Dall ^^^^ states : 



These beds are composed of sandstones and shales and extend northward from Cape Arago 

 to Cape Gregory and thence eastward to Miners Flat, on the south shore of the entrance to 

 Coos Bay, Oregon. They have an average dip of 70° NE. and a thickness of over 3,000 feet. 

 They contain Cardita planicosta, AmpuUina sp., and other middle Eocene forms which suggest 

 their correlation with the Claibornian of the Gulf column. They are apparently newer than 

 the blackish rocks of the region about the junction of Little River and the North Umpqua, 

 which also contain Cardita planicosta but which have also a large proportion of distinct species. 

 They appear to be older than the brackish-water deposits which inclose the Coos Bay lignites, 

 though the latter have occasional intercalary marine layers which contain species of fossil 

 shells apparently identical with members of the Arago fauna. The brackish-water species 

 comprise forms belonging to Corbicula, Cyrena, and Melania or Cerithiopsis, forcibly recalling 

 those which occur in the lignite beds of the Puget group, some of which prove to be identical. 



The Umpqua beds above alluded to contain a notable number of "Loxonema" turrita 

 Gabb, which is stated by him to be common in the Tejon, and have been referred by DiUer ■» 

 to the Tejon group. While this disposition of them may be confirmed by a study of their 

 fauna and is probable, the Arago beds, which were not at first discriminated from those of the 

 iJmpqua, now appear, from the differences in their fauna, to require separation as a distinct 

 series of beds. 



o Bull. Geol. Soc. America, vol. 4, 1893, p. 219. 



