24 GEOLOGY. 



The Triassic in the West. 



The deposits of the western interior. 1 — When general sedimenta- 

 tion ceased in the eastern half of the United States near the close of 

 the Paleozoic era, a tract along the Pacific coast probably remained 

 beneath the sea, while another great area in the western mterior, but 

 partially and temporarily connected with the sea, became the site 

 of varied sedimentation. Between the ocean on the west and this 

 interior area of sedimentation, there seems to have been an elongate 

 area of land which, including much of Mexico on the south, stretched 

 north through western Arizona, eastern Nevada, western Utah, 

 eastern Idaho, and western Montana, to British Columbia. In the 

 United States, the interior area of sedimentation was chiefly between 

 the 100th and the 113th meridians. Its southern limit, so far as now 

 known, was not far from the southern boundary of the United States, 

 while at the north it extended somewhat into Canada. This area 

 of sedimentation is believed to have been cut off from the Gulf by 

 a considerable land area in eastern Texas. If it had connection with 

 the sea at all, as is very doubtful, it was probably slight, and with 

 the Pacific Ocean north of the boundary of the United States. Into 

 this interior area of sedimentation, which perhaps did not depart 

 widely from the area of Permian sedimentation, detritus was borne 

 from the surrounding lands. Some of the deposits were probably 

 laid down subaerially by streams, some in fresh-water lakes, and some 

 in bodies of salt water, as in the Permian period. The structure of 

 some of the sandstone is such as to suggest strongly an eolian origin. 



The deposits of the period are in large measure concealed by later 

 beds, but are exposed at various points where the strata have been 

 elevated, and the overlying beds removed by erosion. The most 

 easterly outcrops of the system are found in Texas, 2 Indian Territory, 3 

 and South Dakota. The Triassic system may underlie the later for- 

 mations west of these localities, and between them and the Rockies. 



1 There is some doubt about the age of most of the beds formerly referred to this 

 system. The tendency of later study has been to refer more and more of them to 

 the Permian. See references under Permian, and Hill, Physical Geography of the 

 Texas Region, folio U. S. Geol. Surv. 



2 See last foot-note. 



3 Gould, Univ. of Kansas Quarterly. 



