JURASSIC PERIOD. 431 



Ocean current alluded to, or to both circumstances combined. The 

 indications of floating ice, which Ramsay has found in the British 

 Lower Permian, may have been a mark of the slow approach of such 

 .an era of cold. 



2. JURASSIC PERIOD (17). 



The Jurassic period derives its name from the Jura Mountains, on 

 the western borders of Switzerland, one of the regions characterized 

 by the formation. 



1. American. 



I. Rocks : kinds and distribution. 



On the Atlantic Border, the upper portion of the formation described 

 in the preceding pages, on the Triassic, may belong, as has been ob- 

 served, to the Jurassic period. The absence of marine fossils leaves 

 the question in doubt. 



On the Gulf Border, there are no rocks of this period anywhere ex- 

 posed to view. 



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

 — perhaps a large part — of the gypsiferous beds referred to the Tri- 

 assic ; but fossils are here also wanting. 



Apart from these doubtful beds, there are true Jurassic strata, full 

 of marine fossils, overlying in many places the gypsiferous marlytes and 

 sandstone. They have been observed about the Black Hills, the Lara- 

 mie range, and other eastern ridges of the Rocky Mountains ; also 

 over the Pacific slope, in the Uintah, Wahsatch, and Humboldt moun- 

 tains, and in the Sierra Nevada. Whitney has found that Jurassic 

 dossils occur in auriferous slates of the Sierra. 



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

 iferous 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 (43J° 

 IS., 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.) Other localities are near the valley of 

 Green River, east of the Great Salt Lake, as announced by Meek & Engelmann; and 

 near Fort Hall, in Idaho. 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 north- 

 east side of the Wind River Mountains, 800 to 1,000 feet; about Long's Peak, where 

 the marlytes are absent, 50 to 100 feet. Another region of Jurassic rocks, on the 

 north slope of the Uintah Mountains, has been described by Marsh. The rock is lime- 

 stone (containing species of Trigonia, Camptonectes, Chemnitzia, etc.), overlaid by red 

 gypsiferous beds, sandstone, and red and gray shales (containing Belemnitts densus), in 



