28 GEOLOGY. 



have supplied such a volume of sediment, the land to the east must 

 have been high, or repeatedly renewed, to counterbalance the waste, 

 unless the high measurement of thickness be due to oblique deposition. 



In the western region, where the system has its greatest thickness, 

 two principal divisions have been recognized, viz., the Koipeto below 

 (4000 to 6000 feet thick), and the Star Peak above (10,000 feet). The 

 lower of these series consists of siliceous and argillaceous beds, and 

 the upper of sandstone, quartzite, and limestone. In the mountains, 

 such as the West Humboldt range of Nevada, the system, especially 

 the lower part of it, is highly metamorphic, and more or less affected 

 by irruptive rocks. 



Farther west, Triassic rocks, now upturned and eroded, are exposed 

 near the summit of the Sierras l in northern California (Plumas County), 

 and at various points northward to Alaska. Recently an extensive 

 series of marine Triassic beds has been identified in the Eagle Creek 

 Mountains of northeastern Oregon, 2 and in the Snake River canyon 

 between Oregon and Idaho. In the northern part of the United 

 States, the Triassic beds, if as wide-spread as the above occurrence 

 suggests, are largely concealed by igneous rocks and by sedimentary 

 beds of lesser age. 3 West of the Gold Range in British Columbia, 

 Triassic formations (Nicola), largely igneous, are wide-spread and 

 thick (13,500 feet). Locally, at least, the system is unconformable on 

 the Carboniferous. 4 The igneous intrusions are thought to be largely 

 submarine. 5 The Triassic is also known in Vancouver and Queen 

 Charlotte Islands. Igneous formations of Triassic age are thought 

 to be wide-spread in southeastern Alaska. 6 



Though most of Mexico appears to have been land during the Tri- 

 assic era, there were within its area (Sonora) inclosed bodies of water 

 as in the United States. The estuary or inland-sea phase of the for- 

 mation also appears in Central America. 



The succession of faunas in the Trias of the Pacific coast indicates 



1 Geol. Surv. of California. 



2 Lindgren, Sci., Vol. XIII, N. S., 1901, p. 270. 



3 For details of the Trias (Jura-Trias) on the Pacific coast, see the following folios 

 of the U. S. Geol. Surv.: Truckee, Bidwell Bar, Jackson, Lassen's Peak, P3'ramid 

 Peak, Mother Lode, and Sonora, Cal., and Roseburg, Ore. 



4 Dawson, Bull. Geol. Soc. of Am., Vol. XII, p. 72. 

 6 Dawson, Science, Mar. 15, 1901. 



Brooks, Bull. Geol. Soc. of Am., Vol. XIII, pp. 260-3. 



