THE COMANCHEAN PERIOD 731 



the result of the physical conditions where the sedimentation took 

 place, conditions which might have existed along the lower courses 

 of rivers or at their debouchures, where shore-waters had little 

 effect upon them. 



In addition to the clastic sediment, there is a little lignite, and 

 some iron ore, and though both are widely distributed, neither 

 is of much commercial value. 



Stratigraphy and stratigraphic relations. The Potomac and 

 Tuscaloosa series are nearly horizontal, with a gentle dip seaward. 

 The Potomac series rests unconformably on Triassic and other 

 formations (Fig. 499), and the Tuscaloosa series on Paleozoic or 



Fig. 499. Section showing relations of various members of the Coastal 

 series. C, Comanchean; K, Cretaceous; E, Eocene; M, Miocene; 

 PI, Pliocene; Q, Quaternary. 



older strata. Both series are overlain unconformably by the Upper 

 Cretaceous. 



Thickness. The Potomac series rarely reaches a thickness of 

 700 feet. The thickness of the Tuscaloosa series is about twice 

 as great. 



The Texas Region 1 



The Comanchean system is more fully represented in Texas 

 than farther east, but its stratigraphic relations are the same. The 

 beds appear at the surface over an area distant from the coast, dip 

 seaward at a low angle, and are concealed near the coast by younger 

 formations. The system includes three distinct series, (1) the 

 Trinity, (2) the Fredericksburg, and (3) the Washita. The first 

 was perhaps contemporaneous with the Potomac series; and the 

 last is probably younger than any part of the system on the Atlantic 

 coast. The system here is much thicker than farther east, ranging 

 from 1,000 feet to about 4,000. Some parts of it are marine, and 



1 Hill, 21st Ann. Kept., U. S. Geol. Surv., Pt. VII, and Bull. Geol. Soc. Am., 

 Vol. V, 1893, pp. 297-338. Also Hill and Vaughan, 18th Ann. Kept., U. S. 

 Geol. Surv., Pt. II, pp. 217-237. 



