THE COMANCHEAN PERIOD. 125 



extensive than those which occurred in the midst of any one of the 

 Paleozoic periods as now defined, if the Mississippian and Pennsylvanian 

 be regarded as separate periods. To appreciate the force of this point 

 in its bearing on the distinctness of the Early and Later Cretaceous 

 periods, it is needful to anticipate the history of the latter sufficiently 

 to say that it was inaugurated by a notable submergence, affecting 

 great areas. It brought the Atlantic and Gulf coastal plains beneath 

 the sea, allowing (Upper) Cretaceous beds of marine origin to be deposited 

 on the eroded surfaces of the Potomac, the Tuscaloosa, and the Coman- 

 chean series. In Texas, no species of marine life is known to have 

 lived over the time-interval recorded by the unconformity between 

 the two systems. Not only was the Texan area of the Comanchean series 

 submerged, but the waters extended themselves far beyond their 

 earlier limits, covering hundreds of thousands of square miles which 

 had long been land. On the Pacific coast of the United States, the 

 seas of the Late Cretaceous period extended farther east than during 

 the Comanchean period in some places, for the Upper Cretaceous strata 

 sometimes rest on pre-Cretaceous beds. 1 In British Columbia, the 

 reverse was the case, while in some parts of Alaska, the Upper Creta- 

 ceous is unconformable on the Lower. 2 On stratigraphic grounds, 

 therefore, the distinctness of the Lower Cretaceous from the Upper 

 in North America is such as to warrant their recognition as separate 

 systems, and their distinctness in some other continents is perhaps 

 equally great. It is for these reasons that the Lower Cretaceous 

 is here regarded as a system distinct from the Upper. 



Thicknesses of strata afford no basis for the separation of systems, 

 yet it may be noted that though the average thickness of the Coman- 

 chean system is not so great as the average thickness of the formations 

 of most Paleozoic periods, yet its maximum known thickness (26,000 

 feet in California, measured by the customary method) is greater than 

 that which any Paleozoic system is known to possess at any point in 

 America. 



The Lower Cretaceous in Other Continents. 



Toward the close of the Jurassic period, it will be recalled, con- 

 siderable areas of central Europe which had been submerged were 



1 Fairbanks, Am. Jour. Sci., XLV, 1893, p. 478. 

 2 Schrader, Bull. Geol. Soc. Am., Vol. 13, Plate 40. 



