246 C. W. TOMLINSON 



Arizona, and southeastern California, or farther north in the Great 

 Basin, where no sediments contemporaneous with those in question 

 are known to occur. 1 The absence of sediments between the 

 Mississippian and the Cretaceous in the El Paso quadrangle 2 in 

 western Texas may be due altogether to erosion following deforma- 

 tion at the close of the Jurassic period, but this gap in the record 

 makes it possible that land may have existed even here during Red 

 Beds times. 



Richardson has concluded from his studies of the Black Hills 

 Red Beds 3 that those sediments were derived chiefly from the 

 Rocky Mountain area to the southwest and west. West of central 

 Wyoming, the Red Beds group thickens, and the quantity of lime- 

 stone and gypsum in it diminishes, continuously westward across 

 the Idaho border, suggesting a source of sediments in that direction. 

 The great thickness of the group all along the Wasatch Range, 

 wherever it is exposed, extends this suggestion to include a con- 

 siderable land area trending north and south from southern Idaho 

 into central Utah. 



Significance of non-clastic sediments. — The more important 

 limestone members of the Red Beds record the existence of exten- 

 sive bodies of clear and not excessively salty water during parts of 

 the Pennsylvanian period in central Texas, in the Plateau Province, 

 in the San Juan region, and in southeastern W T yoming; during 

 the Permian, in the region north and east of Great Salt Lake (in 

 the early part of the Permian, marine deposition throughout much 

 of Wyoming, prior to the initiation of Red Beds sedimentation 

 there), and in western Texas; and in the Triassic, in northeastern 

 Arizona. 



1 Cf . paleogeographic maps by the following authors: T. C. Chamberlin and 

 R. D. Salisbury, Geology (New York: Henry Holt & Co., 1909), II, 545; III, 3 and 

 62; W. B. Scott, An Introduction to Geology (New York: Macmillan, 1909), pp. 616, 

 662; Charles Schuchert, " Paleogeography of North America," Bull. Geol. Soc. America, 

 XX (1909), Pis. 84-88 inclusive. 



2 G. B. Richardson, El Paso Folio (No. 166), Geol. Atlas of the U.S. U.S. Geol. 

 Survey, 1909. 



3 G. B. Richardson, "The Upper Red Beds of the Black Hills," Jour. Geo!., XI 

 (1903), 365-93. 



