THE PERMIAN PERIOD. 623 



In Texas and the region south of the Ouachita uplift, the system 

 has its greatest development. In the Staked Plains (Llano Estacado) 

 it attains a thickness of 7000 feet. The oldest formation of the system 

 (the Wichita) indicates that the critical attitude which characterized 

 the surface farther east during the Pennsylvanian period, now affected 

 Texas, for the beds are partly of marine and partly of fresh-water 

 origin. These beds are succeeded by a formation of limestone {Clear 

 Fork), which indicates a submergence sufficient to allow free entrance 

 of the sea. Advance of the sea is further indicated by the fact that 

 the Middle Permian beds overlap the Lower, and rest upon the Pennsyl- 

 vanian, where no Lower Permian intervenes. The Upper Permian 

 {Double Mountain) which follows indicates a reversal of relations, 

 for much of Texas was again cut off from the ocean and converted 

 into an inland sea, or into inland seas, in which the usual phases of 

 deposition under arid conditions took place. Occasional beds of 

 limestone with marine fossils point to occasional incursions of the sea, 

 while the deposits of salt and gypsum point with equal clearness to 

 its absence, or to restricted connections. 1 In the Guadalupe mountains 

 of western Texas, the Permian system has a thickness of 4000 to 5000 

 feet, and carries faunas unlike those of other regions of North Amer- 

 ica, but similar to those of the late Paleozoic of Asia. 2 



The Permian sea which overspread part of Texas was perhaps 

 continuous to the west, for a part of the time, as far as the Basin region 

 (Utah); but if so, the continuity of the beds has since been inter- 

 rupted by erosion. Fig. 288 shows the manner in which the Permian 

 now occurs in some parts of the plateau region of the west. Beds 

 thought to be Permian are known in New Mexico, 3 in northern Arizona, 4 

 and at other points in the surrounding territory, and in the Wasatch 

 mountains, but not farther west. Throughout much of the area west 

 of the Rocky mountains, the Permian has not generally been differen- 

 tiated. If it is present, as it doubtless is in some places, it has been 

 classed either with the Carboniferous below or with the Trias above. 

 There is often perfect conformity between the Carboniferous below 

 and the beds classed as Trias above, suggesting the presence of unsep- 



1 For Permian of Texas, see Cummins, Geol. Surv. of Texas, 2d Ann. Rept., pp. 

 394-424. Ibid., 4th Ann. Rept., 212-32. 



2 Girty, Am. Jour. Sci., Vol. XIV, 1902, pp. 363-368. 



s Herrick, Jour. Geol., Vol. VIII, pp. 112-125; and Am. Geol., Vol. XXXI, p. 76. 



4 Walcott, Am. Jour. Sci., Vol. XX, 1880, p. 221. 



