13G GEOLOGICAL SURVEY OF THE TERRITORIES. 



wide area, in which the strata almost entirely belong to No. 2 and No. 3. 

 There are two parallel lines of bluffs, the first composed of black 

 shales, which are also shown along the course of the North Fork. Some 

 of the upper beds may belong to No. 4. 



The following is a partial section on a line between station 38 and sta- 

 tion 80. 



Section No. 15. 



Thickness. 

 Base. Ft. In. 



1. Trachyte, about 75 feet thick 



2. Gray and black argillaceous, with Inoceramus 58 



.3. Shales and bluish slates 100 



4. Slope covered with debris of shales above 32 6 



5. Bhiish slates weathering rust-color, in laminae \ of an inch thickness, con- 



taining Inoceramus and fish-scales, too indistinct for identification 160 



6. Gray and grayish-blue limestone with interlaminated shales, containing Jwo- 



ceramus and fish-scales 160 



7. Dark bluish and black shales 243 



Total, about . - 753 6 



At the base of the mesa, which ends east of the Gunnison below its 

 month, No. 2 and No. 3 outcrop. The following section was made north 

 of the Gunnison, opposite the mouth of Eoubideau's Creek. It will give 

 the characters of the beds. They may all belong to the Fort Benton 

 grou{), or the upper portion of the section may possibly represent a part 

 of the Niobrara division. 



Section No. 16. — Gunnison River, opposite Eoubideau's Greelc. 



Thickness. 

 Base. • Ft. In. 



1. Shaly sandstones, with interlaminated argillaceous beds extending frorn"^ 



the top of the blutf on the river to the base of the first bluff north of the | 



river. The beds are for the most part concealed } 175 



2. Coarse yellow sandstone, with calcite 



3. Gray laminated sandstone J 



4. Thin laminae of grayish sandstone shales, with fine black argillaceous 



shales, gypsiferous and calcareous, containing Inoceramus, Ostrea luguhris 



( Conrad), and other Cretaceous fossils 125 6 



5. Yellowish sandstone shales, with quantities of Jwocej-amits and Ostrea espe- 



cially abundant near the top, where there is a layer of black shales 40 



6. Black argillaceous shales, partly concealed by debris - 38 



7. Coarse yellow, gypsiferous, and calcareous sandstones, with layer at top, 

 * breaking into pencil-like pieces one to two inches long and an eighth of 



an inch in thickness 50 3 



8. Sandstone shales ( j20 



9. Fine black ai'gillaceous shales ) 



10. Coarse yellow calcareoussandstone, resembling that described under No. 7. 1 



11. Fine black argillaceous shales, with bands of sandstone (fossiliferous), spe- 



cies of J/ioceramMS and Ostrea 36 



12. Hard gray sandstone 1 



13. Very fine, soft, black argillaceous shales, with a few laminse of gray sand- 



stone. In the lower part of the group the shales are coal-black, but as 

 we ascend they become gray-black. Nearly all the layers are fossilifer- 

 ous. Among the forms are Prioncuclus wyomingensis, Scaphit.es warren- 

 ana, and Inoceramus prohlematicus. They are especially abundant near 

 the top 66 6 



14. Fine gray and yellowish shales • - 34 



15. About 17 feet of gravel, composed largely of basaltic bowlders, forming the 



top of the bluff 



Total, about ,-- 687 3 



Near station 73, on the Gunnison, shales belonging to formation No. 

 2, outcrop beneath the breccia that underlies the rhyolitic covered 

 mesd. (c, Fig. 1, Plate XI\^.) 



