728 GEOLOGY 



of the earlier and lesser submergence, and to the latter, those of the 

 later and more extensive submergence. The distinctness of the 

 Lower and Upper Cretaceous is however so great, that it seems 

 more in keeping with the spirit of modern classification to regard 

 them as separate systems, and the corresponding divisions of time 

 as periods. What was formerly called the Lower Cretaceous series 

 is here called the Comanchean system. The propriety of this 

 classification is the more striking, since it is applicable to other 

 continents as well as to our own. 



FORMATIONS AND PHYSICAL HISTORY 

 The Atlantic and Gulf Border Regions 



That part of the Comanchean system along the Atlantic coast 

 is called the Potomac l series; the part along the eastern Gulf coast, 

 where conditions of sedimentation appear to have been similar to 

 those along the Atlantic, is the Tuscaloosa 2 series. The approxi- 

 mate surface distribution of these series is represented in Fig. 498, 

 which shows that the system outcrops near the inland margin of 

 the Coastal Plain. It is indeed the lowest member of the Coastal 

 Plain group of formations. Neither the Potomac nor the Tusca- 

 loosa series is believed to represent the whole of the period, and 

 the two are not strictly contemporaneous. 



Conditions of origin, and constitution. 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, so that little warping of the surface appears bo 

 have been needed to convert portions of the coastal lands into 

 sites of deposition, though more may have been necessary to provide 

 lands high enough to furnish abundant sediments. The penepla- 

 nation of the eastern mountain and plateau region (luring the ,Ju 

 sic period was no doubt attended by deep decay of the underlying 



'McGee, Am. Jour. Sci., Vol. XXXV, 1888, pp. 120-143; Clark ami 

 Bibbins, Jour. Geol., Vol. V, 1897, pp. 479-500, Hull. (leol. Soc. Am., Vol. 

 XIII, pp. 187-214j also reports of the Geological Surveys of NY\v Jersey aixl 

 Maryland, and folios of the U.S. Geol. Surv. covering parts of the Atlaiilic 

 Coastal Plain. 



2 Smith, Geol. Surv. of Ala., 1894. 



