Geol.-Vol. I.] SMITH— COMPARATIVE STRATIGRAPHY. 347 



Red Beds. These extend in a fringe around the marine 

 sediments from the Grand Canon region on the south, 

 along the Rocky Mountains, up into British Columbia. 



As the sea retreated westward the brackish water basins 

 followed it; thus the Triassic Red Beds do not all belong 

 to one horizon, but take a successively higher place in the 

 column towards the west. In Oklahoma the Red Beds 

 contain Permian fossils of brackish water origin ; in north- 

 western Texas they contain fresh water Triassic fossils. In 

 southeastern Idaho the marine sediments of Lower Trias- 

 sic age are overlain by barren red sandstones representing 

 the Middle Trias. And in northern California, after the 

 Hosselkus limestone epoch, a series of sandstones were 

 deposited, containing only fossil plants of Rhaetic age. 

 Further than this the encroachment did not go, for in cen- 

 tral Nevada and northern California the next epoch, the 

 Lias, is characterized by a marine fauna. 



A very similar epicontinental sea has been traced out by 

 W. N. Logan (17) for the Middle Jura, showing a subsid- 

 ence and transgression of the interior sea over part of the 

 area covered during the Lower Trias. 



That these Triassic sediments were laid down in an arm 

 of the greater ocean, and not in a closed basin analogous 

 to the Caspian Sea of to-day, is shown by the fact that 

 their successive faunas show a close relation to forms that 

 existed contemporaneously in other regions bordering on 

 the Pacific Ocean, and in the ancient Mediterranean Sea, 

 or Tethys, which in Mesozoic time covered a large part of 

 southern Asia. 



Near the close of the Trias we see the culmination of 

 that progressive elevation of the land that began in the 

 Mississippi Valley region at the beginning of the Coal 

 Measures, and extended gradually across the American 

 continent until all that was left of the great interior sea 

 was merely a small gulf, a few hundred miles in extent. 

 This adds another chapter to the remarkably uniform his- 

 tory of North America which has been recorded in the 

 rhythmical advance and retreat of the sea across its surface 



