THE COMANCHEAN PERIOD. 107 



mediterranean sea several hundred miles wide from the mouth of the 

 Mackenzie River on the north, to the mouth of the Rio Grande on the 

 south, dividing the continent into two unequal parts, a larger eastern, 

 and a smaller western. On the Pacific coast also, the sea extended 

 its area somewhat at the expense of the land. There have been few 

 greater incursions of the sea over the land, and therefore few equally 

 great geographic changes, during the long history of the North Ameri- 

 can continent. A long period of deposition was initiated by the sub- 

 mergence, and this was succeeded in turn by (4) a widespread with- 

 drawal of the waters. The mediterranean sea disappeared, and the 

 borders of the land were extended seaward on the east, the south and 

 the west, and the continent became nearly or quite as large as now. 



The formations of the Cretaceous system are commonly divided 

 into two main series, the Lower and Upper. To the former are 

 referred the deposits 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, on the whole, in keeping with the spirit of the classi- 

 fication here adopted, to regard the two series as separate systems, 

 and the corresponding divisions of time as separate periods. From 

 the physical standpoint, the distinction between the Upper and Lower 

 Cretaceous is greater than that between the different parts of any Paleo- 

 zoic system, as commonly classified, if the Mississippian and the Penn- 

 sylvanian be regarded as separate systems, and greater than that between 

 the Cambrian and the Ordovician, or between the Devonian and 

 Mississippian. The paleontological phase of the question is discussed 

 elsewhere. If the Lower Cretaceous be separated from the Upper, 

 it may be called the Comanchean or Shastan system. 1 The propriety 

 of this classification becomes the more striking, since it is equally 

 applicable to other continents. 



This classification involves no new idea. Hill, who has made a 

 special study of the North American Cretaceous where both the Lower 

 and Upper systems are developed, has repeatedly emphasized their 

 distinctness, 1 and Neumayr, 2 after reviewing the relevant evidence 



1 The first of these terms has been applied to the Lower Cretaceous of Texas (Hill), 

 and the second, by Le Conte and others, to the Lower Cretaceous of California. 



2 See references to his papers in the following pages. 



3 Erdegeschichte Bd. II, p. 377. 



