THE COMANCHEAN PERIOD. 115 



Paleozoic strata. Both the Potomac and Tuscaloosa are overlain 

 unconformably by the Upper Cretaceous beds. 



Thickness. — The Potomac series rarely reaches a thickness of 700 

 feet, while the thickness of the Tuscaloosa series in Georgia, Alabama, 

 and Mississippi reaches 1000 to 1500 feet. 



Fig. 380. — Section showing relations of various members of the Coastal series. C, 

 Comanchean; K, Cretaceous; E, Eocene; M, Miocene; PI, Pliocene; Q. Quaternary. 



The Texas Region. 1 



The Lower Cretaceous system is much more fully represented in 

 Texas than farther east and north, but its stratigraphic relations 

 are the same. The beds appear at the surface over an area distant 

 from the coast (Fig. 380), dip seaward at a low angle, and are concealed 

 near the coast by younger formations. 



The Comanchean sj T stem of Texas embraces three distinct series. 

 The oldest was perhaps contemporaneous with the Potomac series, 

 but the youngest is probably younger than any part of the series of 

 the Atlantic Coast. The system is much thicker in Texas than far- 

 ther east, ranging from 1000 to about 4000 feet, the slighter thickness 

 being to the northeast, and the greater to the southwest. In Mexico, 

 these thicknesses are greatly exceeded. 



The three series of the Comanchean system, commencing below, 

 are (1) the Trinity, (2) the Fredericksburg, and (3) the Washita. 



1 Present knowledge of the Cretaceous in this region is due largely to the work 

 of R. T. Hill. The latest account published is in the 21st Ann. Rept. U. S. Geol. 

 Surv., Pt. VII. An earlier paper, Geology of Parts of Texas, Indian Territory and Ar- 

 kansas adjacent to the Red River, Bull. Geol. Soc. Am., Vol. V, 1893, pp. 297-338, con- 

 tains a list of the author's other papers, the more important of which, from the pres- 

 ent point of view, are the following: The Texas Section of the American Cretaceous, 

 Am. Jour, of Sci., Vol. 34, 1887; The Topography of the Cross Timbers and Surround- 

 ing Regions of Northern Texas, Idem, Vol. 33, 1887; Description of the Cretaceous Rocks 

 of Texas and their Economic Value, First Ann. Rept. Geol. Surv. of Texas, 1888; Meso- 

 zoic Geology of Southwestern Arkansas, Ann. Rept. Geol. Surv. of Arkansas, 1888; 

 The Comanche Series of the Arkansas-Texas Region, Bull. Geol. Soc. Am., Vol. II, 1890; 

 Note on the Texas-Neiv Mexican Region, Idem., Vol. Ill, 1891. 



Further accounts of the Cretaceous of Texas are to be found in the Second Ann. 

 Rept. Geol. Surv. of Texas (Taff), and in the 18th Ann. Rept. U. S. Geol. Surv., Pt. 

 II, pp. 217-237, Hill and Vaughan. 



