CENOZOIC ERA. AGE OF MAMMALS. 365 



which contains not more than 5 to 10 or 15 per cent, of its 

 shells still living in neighboring seas or lakes, we call it 

 Eocene; if 15 to 40 or 50 per cent., we call it Miocene ; if 

 50 to 80 or 90 per cent., we call it Pliocene. This is 

 graphically illustrated in the diagram (Fig. 325). 



( Pliocene, 50 to 90 per cent, living. 

 Tertiary Period, j Miocene, 15 " 50 " 

 ( Eocene, 5 " 15 " 



Rock-System. 



Areas in the United States. 1. On the Atlantic 

 "border, going south, we find no Tertiary until we reach 

 New Jersey. Thence to Georgia there is a band of Ter- 

 tiary strata about a hundred miles wide, resting in New 

 Jersey on the Cretaceous, but elsewhere against the 

 Archaean gneiss. It constitutes what are called the low 

 countries of the Southern Atlantic States. The rivers, 

 in passing from the gneissic to the softer Tertiary, make 

 falls or rapids. Here, therefore, is the head of naviga- 

 tion of the Southern rivers, and, therefore, also the posi- 

 tion of many important towns. Eichmond and Peters- 

 burg, Virginia ; Kaleigh, North Carolina ; Columbia, 

 South Carolina ; Augusta, Milledgeville, and Macon, 

 Georgia are thus situated. 



2. The same broad strip of Tertiary lowlands borders 

 the Gulf, resting there, however, on the Cretaceous (see 

 map, page 272), expands northward to the mouth of the 

 Ohio River, and sweeps southward about the western bor- 

 der of the Gulf into Mexico. 



3. On the Pacific border we find Tertiary with Creta- 

 ceous, forming the Coast ranges of California and Ore- 

 gon. All these border Tertiaries Atlantic, Gulf, and 

 Pacific are marine deposits. 



4. But in the interior regions i. e., Plains, Plateau, 

 and Basin we have extensive fresh-water deposits. Some 



