300 It. T. Hill — Texas Section of American Cretaceous. 



the series. The strata dip rapidly to the southeastward. 

 While resembling the Rotten limestone of the Black Prairie 

 region in color, a fact which has prevented its differentia- 

 tion by previous observers, they are entirely different in hard- 

 ness, structure, stratification, and all other respects*. The 

 strata consist of white and yellow limestones, greatly varying 

 in hardness, somewhat subcrystalline or chalky in places, and 

 under a lower power they are seen to be composed of minute 

 loosely-cemented calcite crystals and occasional Foraminifera. 

 Toward the basal portion there are some thin bands of firm 

 gray crystalline limestones, much resembling quartzite in 

 firmness and opalescence. Between the limestone strata there 

 are usually white calcareous marls or shales, of nearly the 

 same color. Sometimes there are horizons of flint nodules, 

 and rarely beds of pulverulent chalk. The thickness of the 

 individual strata vary from over a hundred feet at the mouth 

 of the Pecos river, to less than one foot in the vicinity 

 of Fort Worth and Denison. The rock structure of this 

 lower formation, although crumbling far more readily, does 

 not yield as easily to solution as does that of the Upper Cre- 

 taceous of the Black Prairie region. It breaks into fragments 

 under the expansion and contraction incident to daily ex- 

 tremes of temperature, to which fact is due the rugged 

 outlines of the buttes, mesas and canons in the central or de- 

 nuded region as opposed to the gentle undulations of the Black 

 Prairie region where the exposures of this formation in Texas 

 are found. 



The Central Denuded Region. 



The borders of the central region in which the Comanche 

 series is found present varying sharpness. The northern half 

 is bounded upon the west by the westwardly receding line of 

 the Llano estacado, and upon the east by the eastwardly reced- 

 ing line of the Rotten limestone of the TJpper Cretaceous of the 

 Black Prairie region. This recession of the borders in opposite 

 directions is not apparent in the southern half of the region, 

 where the denudation has not degraded and removed the 

 harder limestones which constitute the eastward faces, scarp and 

 plateau of the Austin New Braunfels non-conformity of the 

 eastern boundary to the level of the Black Prairie region, as 

 has been done along the northern portion of the same edge, 

 while the former scarp of the staked plains has been entirely 

 removed along the southern half of the western border. 



* Governor Roberts, although utterly unaware of the geologic relation, is 

 the first man who clearly noted this difference and fitly distinguished the two 

 regions as the "soft" and the "hard" lime rock regions, respectively. See 

 " A Description of Texas; its Advantages and Resources," by Oran M. Roberts. 

 St. Louis, 1881. 



