748 HISTORICAL GEOLOGY. 



that volcanic forces were vigorously active, not only during a portion of the later Trias, 

 but also in the earlier Carboniferous and later Jurassic. 



The Trias was first recognized as existing probably in Sonora, Mexico, by A. Esmond 

 (J. D. Whitney, Am. Jotir. Sc, 1866). He speaks of it as consisting of sand.stones and 

 conglomerates with coal-bearing clay shales. He adds that the metamorphic slates of the 

 Altar and Magdalena districts, whicli include the richest gold placers of Sonora, may 

 possibly be of Triassic age, but that it is also possible that they are Jurassic, as they 

 •' resemble rather the Jurassic gold-bearing slates of the Sierra Nevada." 



Jurassic. 



Jurassic beds are found at the west base of the Black Hills in Dakota, 

 where the rock is limestone with intercalated marls. The thickness, 200 

 feet, increases to 600 feet 40 miles from the Hills (iSTewton), indicating, as 

 W. 0. Crosby implies, less subsidence in the sea-bottom about the Archaean 

 center than at a distance from it. They also come out to view at points 

 along the base of the Laramie Mountains, the Big Horn Mountains, the Wind 

 River, and other mountains in the Eocky chain. They overlie Triassic 

 through much of the Summit Region within the United States, both east of 

 the Great Basin or Plateau belt, and, as has been mentioned, along its 

 western border beyond 117^°. Farther north in the same belt, they have 

 been observed by Diller on the Blue Mountains of Oregon. 



The Upper Jurassic in Colorado, Wyoming, and Montana includ&s the 

 freshwater Atlantosaurus beds of Marsh, from 100 to 300 feet thick, which 

 have afforded, near Morrison and Canon City in Colorado and elsewhere, the 

 remains of many large Reptiles, teeth and jaws of Marsupial and Oviparous 

 Mammals. The Baptanodon beds of Marsh, when present, are next below. 

 They contain remains of large aquatic Reptiles, besides some marine inverte- 

 brate fossils. 



The Jurassic beds are found along a large part of the western slope of 

 the Sierra ISlevada. The first discoveries were made in Plumas County, on 

 the north slope of Genesee valley, by Clarence King, of the Whitney Survey, 

 in 1863. They were afterward discovered in the auriferous slates of the 

 Mariposa region and identified by fossils (Gabb, 1864; Meek, 1865). 



In the Taylorville region in Plumas County, the Jurassic beds, according to Diller and 

 Hyatt, are found to consist of nearly 1500' of sandstones, 10' to 30' of limestones, and 500' 

 of tufa. The series represents, as Hyatt has found from the fossils, the Lias and the 

 Lower and Upper Oolyte. The Upper Oolyte has also been identified by fossils over a 

 wide range of the western slopes of the Sierra, where the rocks are upturned metamorphic 

 slates, hydromica, mica, and siliceous schist, with sandstone, and in some parts, serpentine, 

 and thin beds of crystalline limestone, besides more coarsely crystalline rocks. The belt 

 of slates — which is in general 20 to 25 miles wide — contains the chief part of the gold- 

 bearing veins of quartz, some of which are of great width. Turner describes the Mariposa 

 slates as including much diabase tufa, besides some conglomerates made of siliceous 

 pebbles from the associated rocks (1894). 



The most abundant fossil in the Mariposa beds is a species of Aucella (see beyond, 

 page 760), and hence related beds have been called Aucella beds. The Mariposa rocks 

 were pronounced Jurassic by Gabb (r864) and Meek (1865), and recently also by Hyatt. 



