412 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



EXPLANATION OF PLATE XL. 



Plate xl shows the distribution of marine Triassic sediments and the prob- 

 able distribution of land and sea in the United States during the Trias. 

 Columnar sections of the Trias are given, showing the approximate thickness 

 and age of the marine sediments at the six localities where they have been 

 studied in detail. 



1. Inyo Range, California, with about a thousand feet of calcareous shales 

 and limestones with a typical Lower Triassic fauna, surmounted by a few 

 hundred feet of shales and limestones, with a fauna referred to the base of 

 the Middle Trias. 



2. Santa Ana Mountains, California, showing a few hundred feet of lime- 

 stones with a fauna probably of Lower Triassic age. 



3. Aspen Mountains, Idaho, with about eight hundred feet of sandstones 

 and impure limestones with a typical Lower Triassic fauna. 



4. Humboldt Range, Nevada, showing about eight hundred feet of shaly 

 limestones with a Middle Triassic fauna, and about two thousand feet of 

 calcareous beds of the Upper Trias. 



5. Genesee Valley, Plumas County, California, showing about five hun- 

 dred feet of limestones, with an Upper Triassic fauna. 



6. Shasta County, California, showing about fifteen hundred feet of 

 siliceous shales with Middle Triassic fossils, overlain by about five hundred 

 feet of calcareous sediments with a rich Upper Triassic fauna. 



It is probable that the area shown on the map was mostly covered by sea 

 during the Lower Trias, but that the waters retreated gradually westward, 

 until near the close of the Trias only a small remnant was left. 



