124 JOSEPH E. POGUE 



severely mashed. A typical example of the mica schist is dark- 

 reddish to bluish-gray in color, weathering to an iron-stained sur- 

 face glistening with mica scales along its planes of foliation and 

 presenting on transverse break thin quartzose lenses, deformed 

 survivors of original pebbles. It is only after having traced the 

 Cantwell through its progressive stages of metamorphism from 

 west to east that an observer would recognize this type as the 

 mashed equivalent of the massive conglomerate so extensively 

 developed near the mouth of Jack River, 35 miles to the west. 



Associated igneous rocks. — -In the- western extension of the 

 formation lava flows of andesitic, rhyolitic, and basaltic character 

 are interbedded with the conglomerate, and these intercalations 

 are especially prevalent toward the Nenana Valley, in some locali- 

 ties equaling the sediments in thickness."^ West of the Nenana the 

 Cantwell is cut by dikes of diabase and stocks of granite. Refer- 

 ence has already been made to the granite batholiths that invade 

 the Cantwell sediments east of the Nenana Canyon. Adjacent to 

 these areas the sediments are traversed by abundant dikes of 

 rhyolite porphyry and diorite porphyry, the former especially con- 

 spicuous near the head of Wells Creek. 



Structure. — -The structure of the Cantwell formation has been 

 suggested in the preceding paragraphs. Its low northward dip 

 and unmetamorphosed character on the west, with increasingly 

 steeper dips and development of schistose textures along its eastward 

 course, point to folding, gentle to the west, more and more intense 

 to the east. The trend of the formation, and of the axis of folding, 

 as indicated by numerous strike measurements, as well as by the 

 course of the southern boundary of the formation, is about N. 80° E. 



The inclination of the massive conglomerate beds where the 

 Nenana River turns northward and cuts through them indicates 

 that the southern face of the mountains here represents the beveled 

 upturned edge of a gentle synclinal fold. According to Brooks^ 

 this type of broad open folding holds for the westward extension 

 of the formation. At the head of Wells Creek the Cantwell is 

 structurally more complicated than in the area just noted. It 



^ Brooks, op. cit., p. 79. 



2 Op. cit., p. 79; also section CD on geologic map, Plate 9. 



