70 PRINCIPLES OF STRATIGRAPHY 



Great Salt Lake, the shrunken remnant of Lake Bonneville. In 

 southern California the Mohave and Colorado deserts lie just east 

 of the Coast Range. In the higher parts of the Rocky Mountains 

 these winds again yield some moisture, but beyond these ridges 

 they once more become drying winds, with corresponding semi- 

 aridity of the climate. The winds would continue so, and arid con- 

 ditions would prevail throughout the entire interior of the conti- 

 nent, were it not for the aperiodic cyclonic winds which, passing 

 over the country from east to west, cause moist air to flow from 

 the Gulf northward to cooler latitudes. 



If the south winds from the Gulf were shut out by a mountain 

 range, the region to the northeast of it would likewise be of an arid 

 nature. Such conditions seem to have existed in Triassic time in 

 North America and western Europe. In North America the newly 

 formed Appalachian ranges shut in the land on the Atlantic side, 

 and their westward continuation in Arkansas, Oklahoma, and Texas 

 formed a barrier against the winds from the south. Continuing 

 through the region of the present Basin ranges, as the Palaeo-Cor- 

 dilleran Mountains, they formed the western arm of the system 

 which embraced interior North America. To the south and west 

 extended the Triassic sea with its marine limestone accumulations. 

 Rising over the Palaeo-Cordillerans in the southern region (Arizona, 

 New Mexico), these winds became drying winds, as a result of 

 which semiarid continental deposits were formed on the leeward 

 side of these mountains. These are now found as extensive red 

 sandstone deposits, partly of torrential, but perhaps more largely 

 of eolian character, extending from Texas to Montana and Canada. 

 They are heaviest in the Arizona region and extend eastward and 

 northward into Colorado and Dakota, dipping under the Cretacic 

 deposits of the Great Plains. They come to an end before reach- 

 ing the Mississippi Valley, where Cretacic beds rest disconformably 

 upon Permic. Over the whole eastern half of North America, west 

 of the newly made Appalachian Mountains, no Triassic deposits are 

 known, and erosion rather than deposition seems to have been ac- 

 tive. This erosion probably was in part eolian, similar to that known 

 in the Kalahari desert, but near the mountains precipitation was 

 again abundant, so that river erosion characterized the mountain 

 regions. Much of the drainage from these mountains must have es- 

 caped toward the northwest, where an arm of the sea entered during 

 part of Triassic and later Jurassic time. Some of the continental de- 

 posits formed over the crystalline region of Canada and northern 

 United States may have subsequently been reworked into the con- 



