THE COMANCHEAN PERIOD. Ill 



beds of the Atlantic province were laid down (p. 1). The sedimen- 

 tation was attended and followed by igneous intrusions, and prob- 

 ably by faulting and warping. At the close of the Triassic period, 

 as nearly as now known, the surface was again deformed, and a period 

 of erosion which lasted through the Jurassic period inaugurated. 

 By the beginning of the Comanchean period, both the Appalachian 

 Mountains and the area of the present Piedmont plateau had been 

 degraded well toward base-level. 1 Little warping of the surface there- 

 fore appears to have been needed to convert portions of the coastal 

 lands into sites of sedimentation. That part of the Comanchean 

 (Lower Cretaceous) system which is found along the Atlantic coast is 

 called the Potomac 2 series. The formations tentatively referred to 

 the Jurassic 3 are generally included in this series. Other names 

 have local application (see table above). 



The conditions of sedimentation along the eastern part of the Gulf 

 coast appear to have been similar to those along the Atlantic, and 

 the corresponding formations constitute the Tuscaloosa 4 series. 



The approximate surface distribution of the Potomac and Tusca- 

 loosa series is shown on the accompanying map (Fig. 379), from which 

 it is seen that they are not traceable into each other at the surface; 

 but there is general agreement that they were, at least in part, con- 

 temporaneous. Neither is believed to represent the whole of the 

 Comanchean system as developed elsewhere. On the basis of fossils, 

 the Tuscaloosa is thought to represent only the latter part of the time 

 when the Potomac was in process of deposition, while both are referred 

 to the early rather than the late part of the period. If both are 

 referable to the earlier part of the Comanchean period, it is not now 

 possible to say how far this is to be accounted for by the emergence 

 of the regions where the series occur before the later part of the period, 



1 The possible Jurassic beds of the Atlantic coast (p. 59) are not brought into 

 consideration here. 



2 McGee, Am. Jour. Sci., Vol. XXXV, 1S88, pp. 120-143; Clark and Bibbins, 

 The Stratigraphy of the Potomac Group in Maryland, Jour, of Geol., Vol. V, 1897. 

 pp. 479-506. This article treats of the Potomac as a whole; also Bull. Geol. Soc. 

 Am., Vol. XIII, pp. 187-214, 1902. 



3 Marsh thought the whole Potomac series Jurassic, Am. Jour. Sci., Vol. II, 1896, 

 pp. 433-477. 



4 Smith and Johnson, Bull. 43, U. S. Geol. Surv., 1887. For a better and later 

 summary of the Tuscaloosa of Ala., see Smith, Geol. Surv. Ala., 1894. 



