JURASSIC PERIOD. 445 



In the Western Interior region, the Jurassic period may claim a part 

 — perhaps a large part — of the gypsiferous beds already described: 

 here, again, fossils are wanting to decide the question of age. 



But, apart from these doubtful beds, there are true Jurassic strata 

 full of fossils, overlying in many places the gypsiferous marls and 

 sandstone. They have been observed about the Black Hills and 

 the Laramie Mountains, and also at the base of other ridges in the 

 Rocky Mountains. The beds consist of impure limestone with 

 layers of marl. 



In the Arctic region, also, there are a number of localities of fossili- 

 ferous Jurassic strata. 



The discovery and identification of the Jurassic of the Black Hills of Dakota 

 were made by Hayden & Meek. The rocks occur also at Red Buttes on the 

 North Platte, west of the Black Hills ; also along the southwest side of the 

 Big Horn Mountains (43i°.N., 108° W.), and the northeast side of the Wind River 

 Mountains ; also beyond the Wind River Mountains, on the west ; also about 

 the head-waters of the Missouri : — at all of which places fossils occur. (Hayden.) 

 Another locality is near the valley of Green River, east of Lake Utah (Great 

 Salt Lake), as announced by Meek & Engelmann. 



The rocks observed are in general a gray or whitish marly or arenaceous 

 limestone, with occasional purer compact limestone beds, intercalated with 

 laminated marls. The thickness at the Black Hills is about 200 feet; on the 

 northeast of the Wind River Mountains, 800 to 1000 feet; about Long's Peak, 

 where the marls are absent, 50 to 100 feet. 



The Arctic localities are — the eastern shores of Prince Patrick's Land, in 76° 

 20' N., 117° 20' W. ; the islands Exmouth and Talbe, north of Grinnell Land, 

 77° 10' N., 95° W. ; and Katmai Bay, or Cook's Inlet, in Northwest America, 

 60° N., 151° W. 



n. Life. 



Although but little is yet known of the life in America of the 

 Jurassic period, several genera of Eadiates and Mollusks which 

 mark the Jurassic beds of Europe have here been found, the most 

 prominent of which are Pentacrinus, Trigoyiia, Ammonites, and Be- 

 lemnites. The characteristics of Belemnites and Ammonites are 

 briefly mentioned on p. 156, and again beyond, on pp. 450, 451. 



Characteristic Species. 



No plants have been described, except a few by Newberry from a coal seam in 

 the gypsiferous sandstone of the Upper Colorado, in the Moqui country (near 

 the meridian of 111°), the age of which is doubtful (p. 417). The observed 

 genera are Cyclopteris, Pecopteris, Netiropteris, Sphenopteris, and Clathropteris. 



The Clathropteri8 from near the middle of the Connecticut River sandstone 

 (fig. 628, p. 419), as suggested by Hitchcock, is some evidence — though far from 



