CHAPTER XX 



Transition from Paleozoic to Mesozoic. The Permian 

 period fades into the Triassic so gradually that a dividing 

 line cannot always be drawn between them. In eastern 

 United States, it is true, great geographic changes had taken 

 place, and the crumpling of the Appalachian sediments had 

 produced ranges of mountains where there had been a coastal 

 plain before; but. the rest of the continent remained much 

 as in the Permian period. 



Deserts continue in the West. In the Triassic, as in 

 the Permian, red sediments were deposited where the Great 

 Plains and Rocky Mountains now stand. Beds of gypsum, 

 the relics of salt lakes, point to the prevalence of desert condi- 

 tions, much as in Nevada and Utah to-day. Needless to say, 

 the remains of living things are very rare in the .red beds. It 

 is probable that much of the West was an inhospitable place 

 for both plants and animals, except such hardy forms as were 

 fitted to live in an arid land. 1 



Marine strata on the Pacific slope. Marine rocks of 

 Triassic age are found only on the western border of the 

 continent, and in but few places even there. From this we 

 may infer that the seas which had been drawn off from the 

 continental platform during the Permian were very slow in 

 creeping back upon it. The Pacific is the only ocean known 

 to have encroached upon the North American land during the 

 period. Its shore line lay somewhat farther east than now, 

 especially in British Columbia and Alaska. In the United 

 States it reached Idaho and Nevada. Off this coast thick 



1 It may be noted, in passing, that desert conditions were prevalent also 

 in western Europe at this time, as they had been in the Permian period. 

 B. & B. GEOL. 23 397 



