peaue.! DESCRIPTIVE GEOLOGY UPPER BEAR RIVER VALLEY. 575 



will be again considered. To the southward it ends very soon, and higher 

 beds appear to curve around the end of the ridge over the limestones. 

 On the west side of the ridge the Wahsatch beds, with isolated cappings 

 of Green Eiver shales, rest unconformably on the limestones, and the 

 erosion of this ridge doubtless contributed to the formation of the con- 

 glomeritic Wahsatch beds. The valley of Bock Creek is a monoclinal. 

 It is about 14 miles in length, and not over half a mile in width at the 

 widest portion. It is 1,400 feet below the top of the Carboniferous ridge. 

 On the west side is a rather regular flat-topped ridge that rises 1,000 

 feet above the creek-level. IsTear the southern end of this a substation 

 was located. The beds outcropping on the east face of the ridge were 

 limestones and quartzites. They were all somewhat obscure, but near 

 the top of the ridge was an outcrop of light yellowish-gray limestone, in 

 which I obtained a few indistinct organic remains, identified by Dr. 

 White, as follows : 



JSumicrotis curta ? 



Myalina whiteii 



Aviculopecten idakoensis f 



Myadtes ? 



Modiola f 

 Above these limestones a whitish quartzite forms the summit of the 

 ridge. All the strata dip to the westward. 



At one point there appeared to be a capping of Green Eiver shales 

 in horizontal position, resting on the upturned edges of the Jurassic 

 strata, but it could not be defined as the hill was so thickly covered with 

 debris. Above the quartzites followed limestones and shales, above which 

 were reddish sandstones and shales, all referable to the Jurassic, accord- 

 ing to their j)osition and litkologiCal structure. To the westward these 

 beds disappear beneath the valley of Bear Eiver. Below the mouth of 

 Bock Creek, Twin Creek flows in a broad sage-brush valley. On the 

 north side blue Jurassic limestones outcrop in low bluffs on the edge of 

 the valley, and on them rest Wahsatch sandstones unconformably. 



Station 108 was located south of the valley on a hill, in which there 

 were a few obscure outcrops of Jurassic limestone surrounded by Wah- 

 satch beds. A short distance northwest of the station the valley of Twin 

 Creek narrows as it cuts across a low ridge. At this point there is a 

 coal-mine, owned by the Wyoming Coal and Coking Company. The 

 bed was discovered in 1875. There is a tunnel 470 feet in length, with 

 a side shaft 120 feet in length. The following is the section beginning 

 with the coal-bed penetrated by the tunnel. 



Section No. 23. 



1. Coal, 3* feet. 



2. Shales and slates. 



3. Coal, 3 or 4 inches. 



4. Clay, 6 feet, decreasing in places to 3 feet. 



5. Coal, 6 feet. 



0. Light-colored sandstones forming the roof. 



These beds dip to the westward or a little north of west, at an angle 

 of about 40°. The sandstones are fossihferous. The following were 

 obtained : 



Rhytophora meelxii. 



Corblcula (Veloritina) durlceei. 



Goniobasis chrysaloidea. 



Pyrgulifera humerosa. 



Goniobasis chrysalis. 



Volsella (Brachydontes) — ? 



Ostrea f 1 



Neritlna f 



