490 CENOZOIC TIME. 



3. Pliocene, from 7rAe<W, more, etc. ; more than half the species 

 living. 



In the application of these terms to British and European rocks, they came to repre- 

 sent certain beds in the Tertiary series, and thus to have a significance independent of 

 the precise number of living species represented by the fossil remains. Some geolo- 

 gists make a fourth division, called Oligoctne, by separating an upper portion of the 

 Eocene, and uniting with it the lower section of the Miocene. 



1. American. 



The periods in American Geological history, which are marked 

 off by the breaks in the Tertiary series, are : — 



1. The Lignitic period, or that of the earlier Eocene, an era 

 largely of fresh-water formations, whose beds over the Rocky Moun- 

 tain region lie unconformably beneath those of the next period, a. 

 mountain-making epoch having intervened. 



2. The Alabama period, or that of the Later (Middle and Upper) 

 Eocene, an era of marine formations on the borders of the Atlantic, 

 Mexican Gulf, and Pacific, but ending in a geographical change that 

 excluded later marine Tertiary beds (or those having marine fossils) 

 from Southern Alabama, Mississippi, and Texas, or the borders of the 

 Mexican Gulf. Over the Rocky Mountain slopes and summit, only 

 fresh-water formations. 



3. The Yorktown period, corresponding to the Miocene, or Mio- 

 cene and part of the Pliocene (so named from a locality in Virginia) 

 to which a large part of the beds in view on the Atlantic Border 

 belong. Over the Rocky Mountain slopes and summit, only fresh- 

 water formations. 



4. The Sumter period, supposed to correspond to the Pliocene, 

 or part of it; named from a locality in South Carolina. 



I. Rocks : kinds and distribution. 



The deposits are either of marine or of fresh-water origin. The 



marine indicate the presence of the ocean's waters in the region where 



they occur, and enable us therefore to mark out approximately the 



limits of the oceans over the continents, while the fresh-water beds are 



mostly of lacustrine origin. 



The Tertiary areas on the map, p. 144, are lined obliquely from the left above to the 

 right below; and the fresh and brackish-water Tertiary area, which occurs on the slopes- 

 of the Rocky Mountains, is distinguished from the marine by a more open lining. 



The general distribution of the marine beds is similar to that of the 

 Cretaceous. On the Atlantic Border, the most northerly point is 

 Martha's Vineyard. In New Jersey, and to the south, through Mary- 

 land, Virginia, and the Carolinas, they cover a narrow coast-region ; 

 and, from South Carolina, they spread westward along the Gulf Border, 



