148 GEOLOGY. 



The Colorado series. 1 — The succeeding series records an exten- 

 sive invasion of the North American continent by the sea. The sub- 

 mergence went so far as to establish a connection between the Gulf 

 of Mexico on the south and the Arctic ocean on the north, over the 

 site of the Great plains, thus dividing North America into two parts 

 by a great mediterranean sea. It was probably not before this epoch, 

 and perhaps not until the next, that the exposed Upper Cretaceous 

 series of the Coastal plain began to be deposited, though exact cor- 

 relation of these widely separated series has not yet been made. 



The limits of the mediterranean sea of the Colorado epoch can 

 only be approximately located. The Western limit appears to have 

 extended from northern Mexico, through Arizona, Utah, eastern Idaho, 

 and western Montana into British Columbia, though at the west there 

 were probably many islands, the cores of the present mountain ranges. 

 The Black Hills, however, were probably submerged. 2 The eastern 

 limit of the sea, so far as now known, lay in Minnesota, Iowa, and Kan- 

 sas, east of the limit of the Dakota sandstone. In Minnesota and 

 northern Iowa, outliers of the Colorado formation are found nearly 

 to the Mississippi. To the south, the sea was constricted by the Oua- 

 chita uplift. The area of this uplift probably extended as a penin- 

 sula from Arkansas into Indian Territory and Oklahoma, and the sea 

 passed around its western end. There may have been a connection 

 between the Gulf and the mediterranean sea east of the Ouachita 

 uplift, making the latter an island. 



It is possible that the mediterranean sea of the Colorado epoch 

 extended much farther east in the basin of the Upper Mississippi than 

 is indicated above, for in a few places in Minnesota, Wisconsin, Iowa, 

 Illinois, Missouri, and Indiana there are beds of gravel which represent 

 the remnants of a once widespread formation, most of which has been 

 destroyed. These remnants may be Cretaceous; but, on the other 

 hand, they may equally well be much younger, 3 so far as now known. 

 They are probably not marine. 



Two principal divisions of the Colorado series are recognized, 

 the Benton (chiefly shale) below, and the Niobrara (largely chalk 



1 For subdivisions of this series, see Logan, Jour. Geol., Vol. VII, pp. 83-91. 



2 Newton, Geology of the Black Hills. 



3 Salisbury, Jour, of Geol., Vol. Ill, pp. 655-667. See also Proc. Am. Assoc, for 

 Adv. Sci., 1892. 



