PALEOGEOGRAPHY OF THE MESOZOIC 505 



PALEOGEOGRAPHY OP WESTERN NORTH AMERICA DURING THE MESOZOIC 



INTRODUCTORY 



Let us now apply the methods just stated to the paleogeography of the 

 west coast of North America during Mesozoic time. 



TRIAS SIC TIME 

 (See Figure 3) 



Our knowledge of the west coast Triassic is good only for the States of 

 California. Nevada,, and Oregon, and is due mainly to the work of Prof. 

 J. Perrin Smith. He states that the development of the Triassic of these 

 States is unusually complete, and in thickness compares favorably with 

 that of any other region of marine sedimentation. The deposits are 

 usually calcareous and fairly thick (about 4,000 feet), and increase in 

 volume with the progress of time, facts which are in keeping with the 

 view that, as no mountain ranges were developed along the Pacific border 

 in Permian time, there was no high land present to furnish the adjacent 

 seas with much sand and mud. 



Along the Pacific border of British Columbia, from Vancouver north 

 to the Queen Charlotte Islands, the Triassic, and chiefly the early Upper 

 Triassic, is of great thickness, attaining, according to Dawson, to 13,000 

 feet, of which more than nine-tenths is of submarine volcanic origin. 

 With these materials are interbedded zones of marine sediments, argil- 

 lites, and quartzites that are thin or even absent to the east. The volca- 

 noes of Middle and Upper Triassic time extended from southern Califor- 

 nia into Alaska, and near Mount Saint Elias there are about 4,000 feet 

 of basalts, followed by the same thickness of Upper Triassic limestone 

 (Chitistone) and 2,500 feet of dark shales. 



The Pacific overlap had its widest distribution in the early Upper 

 Triassic, for its deposits occur at various points along the western border 

 of North America and widely over Alaska and Arctic America. In the 

 United States throughout the Eocky Mountain area the Upper Triassic 

 is developed as a continental series of sandy red or variegated shales and 

 cross-bedded sandstones. 



In Alaska along the Pacific border, from at least Mount Saint Elias to 

 the middle of the Aleutian peninsula, a distance of fully 800 miles, the 

 Triassic and older formations were thrown into a folded series of moun- 

 tains and injected by igneous rocks at the close of the Triassic. It seems 

 very probable that the uplift extended across the peninsula of Alaska far 

 southward into British Columbia. Finally, a marked break in sedimenta- 

 tion separates the Jurassic of California from the Triassic — a condition 

 XXXVII — Bull. Geol. Soc. Am., Vol. 27, 1915 



