126 JOSEPH E. POGUE 



Jack River, where calculations based upon aneroid and triangu- 

 lation measurements show a vertical development of about 2,700 

 feet. In the headwater region of the Toklat basin about 25 miles 

 to the west. Brooks^ found that the thickness there exposed is at 

 least 2,000 feet. These figures do not represent the total thickness 

 of sediments as deposited, because the top of the formation is a 

 surface of erosion, from which an uppermost portion of unknown 

 thickness has been removed. 



Age and correlation. — ^The Cantwell formation is assigned to 

 the Tertiary on the basis of plant remains found in some abundance 

 in the Wells Creek area. The following forms were identified by 

 Drs. F. H. Knowlton and Arthur Hollick, who report that the 

 material is of Tertiary age: 



Taxodium tinajorum Heer. 



Taxodium dubium (Sternberg) Heer ? 



Sequoia langsdorfii (Brongniart) Heer ? 



Populus arctica Heer ? 



Daphnogene Kanii Heer. 



Aspidium Heerii Ettingshausen ? 



Ginkgo adiantoides (Unger) Heer ? 



In the report on the Mount McKinley region^ the Cantwell 

 was provisionally referred to the Carboniferous, although its litho- 

 logic resemblance to several occurrences of the Kenai formation 

 (Eocene) was noted and the possibihties of younger age fully recog- 

 nized. The extent and relation of the plant-bearing area sur- 

 rounding upper Wells Creek and the fact that a locality within the 

 western portion of the Cantwell has also furnished Tertiary plant 

 remains showing identical species^ would seem to fairly establish 

 the present age assignment.'' 



^ Op. cit., p. 81. ^ United Stales Geol. Survey, Prof. Paper 70, 1912. 



•^ Brooks, op. cit., p. 82. This occurrence was interpreted as an infaulted block 

 and the fossil evidence was not regarded as applicable to the surrounding formation. 



■• The possibility must be recognized that future detailed work may result in a 

 subdivision of the Cantwell, as in its western portion in particular the members in 

 which conglomerate beds dominate are succeeded by a series of shales and sandstones. 

 It seems likely, however, that these upper members are merely an expression of normal, 

 changing topographic conditions in the region of supply, rather than the result of a 

 distinctly later epoch of sedimentation. In its eastern portion, where observed by 

 the present writer, the formation appears definitely a unit. 



