THE TRIASSIC PERIOD. 27 



Beds rest on the Pennsylvanian conformably, at least the lower portion 

 of the former should probably be assigned to the Permian. In south- 

 western Colorado and eastern Utah, the Trias rests unconformably on 

 older, deformed, unfossiliferous Red Beds, and on strata of Pennsyl- 

 vanian age, and perhaps overlaps even older formations. 1 



Thickness. — In the eastern part of the inland basin, the Triassic 

 system is thin, sometimes no more than 100 feet. To the west it 

 thickens, reaching to 2000 to 2500 feet in the Uinta Mountains, beyond 

 which it again thins and becomes conglomeratic in western Utah. 

 It is on the basis of these characteristics, as well as because of the 

 absence of the system over western Utah and eastern Nevada, that the 

 western limit of the interior basin is believed to have been in the longi- 

 tude of Great Salt Lake. No general subdivisions of the system have 

 been adopted for this region. 



The Triassic system on the Pacific slope. 2 — In the latitude of Nevada, 

 the Pacific seems to have extended eastward over the site of the Sierras 

 to longitude 117° (approximately), as shown by the distribution of 

 the marine Triassic strata. The shore line of the Pacific farther north 

 has not been definitely located. It was probably irregular, and, in 

 general, several degrees farther east than now, well up into British 

 Columbia. Still farther north, between 55° and 60°, the sea is believed 

 to have crossed the entire Cordilleran 3 belt, though this northern bay 

 east of the Rockies was probably not connected freely with the areas 

 of sedimentation in the western interior. 



It is along the Pacific coast that the Triassic system in America 

 has its greatest development. In the United States, the sediments 

 of this part of the system appear to have been derived from the newly 

 uplifted lands to the east. The published measurements assign the sys- 

 tem the great thickness of 17,000 feet (maximum) in the West Hum- 

 boldt range of Nevada, 4 where it rests on pre-Cambrian terranes. To 



1 Cross & Howe. The unconformity i-; s^en near Ouray, in the 7ncompahgre 

 valley, and above Moab, on Grand River. Bull G. S. A., Vol. XVI, p. 447. 



2 King, Geol. Surv. of the 40th Parallel, Vol. I. An account of the Triassic 

 as far west as the Sierras in this latitude. See also the following folios, U. S. Geol. 

 Surv. : Bidwell Bar, Colfax, Downieville, Jackson, Lassen's Peak, Maryville, Mother 

 Lode, Nevada City, Pyramid Peak, San Luis, Sonora, and Truckee, Cal., and Rose- 

 burg, Ore. 



3 Dawson, Science, March 15, 1901. 



4 King, loc. cit. 



