506 R. T. HILL THE COMANCHE SERIES. 



along the base of the remnantal Cretaceous mesas of the Colorado-Brazos 

 divide, in Nolan, Taylor and Mitchell counties. Owing to the unconsoli- 

 dated, pulvereut nature of these sands, they are denuded more rapidly than 

 the overlying or underlying strata. As a result of this rapid denudation, 

 the main area of Trinity exposures north of the Brazos is in a narrow val- 

 ley, seldom exceeding ten miles in width, which extends nearly five hundred 

 miles irregularly northward from the Brazos to the mountains of Indian 

 territory, and from thence eastward to Murfreesboro, Arkansas. This valley 

 is bounded coastward by the escarpment of the more indurated material of 

 the Glen Rose beds. 



The Trinity valley is one of the most marked topographic features of the 

 Arkansas-Texas region. Where the underlying beds are of unconsolidated 

 material, however, as in the red bed region of northwestern Texas, the rem- 

 nantal sands often occur as a thin sheet of loose sand over extensive upland 

 areas, as seen east of Abilene in Taylor county, and in other places. Some 

 of the sand hills along the western escarpment of the plains are also of this 

 nature. This formation, although of limited areal exposure, has a wide 

 range of occurrence along the interior border of the more calcareous beds of 

 the Comanche series from southwestern Arkansas to New Mexico. It is 

 usually absent along the eastern front of the Rocky Mountains from Las 

 Vegas (New Mexico) northward, unless the Atlaiitosaurus beds at Canon 

 City (Colorado) are synchronous in age, which is not proven. The pink 

 grits at Gallisteo (New Mexico), described by Marcou and Stevenson, and 

 which occur northeast of Santa Fe at Rowe, and at other points near the 

 intersection of the Pecos river, are probably of this formation, and may mark 

 its western border. Upon careful comparison I am also inclined to think 

 the upper half of Tiicumcari mesa, New Mexico, which I have visited, is com- 

 posed, below the cap rock, of the Trinity sands. Traces of this terrane are 

 also seen between the Pecos and the escarpment of the Llano Estacado, in 

 southeastern New Mexico, east of Eddy, indicating its extent beneath the 

 Tertiary plains. 



In southern Kansas the Cheyenne sandstones have been properly ascribed 

 to this age by Cragin, but I am inclined to think froin the anomalies of occur- 

 rence there that they are not of continuous sedimentation with that of the 

 main Trinity sea, but were deposited in an embayment or inlet around the 

 western end of a buried mountain system, of which the Wichitas are now the 

 visible remnant. 



South of the Colorado and east of the Pecos the occurrence and extent of 

 the sands have not been determined ; and, after many journe3^s in northern 

 Mexico and southern Texas, I have been unable to find the base of the 

 Comanche limestones exposed in this region. 



The origin of the Trinity sediments is apparent. They are always derived 



