TERTIARY PERIOD. 507 



forty to sixty per cent, of the fossils are living species, — called 

 also Pliocene. 



I. Rocks : kinds and distribution. 



The Tertiary areas on the map, p. 133, are lined obliquely from 

 the left above to the right below ; and the fresh- and brackish- 

 water Tertiary which occurs on the slopes of the Rocky Mountains 

 is distinguished from the marine by a more open lining. 



The general distribution of the beds is similar to that of the Cre- 

 taceous. On the Atlantic border the most northerly point is Martha's 

 Vineyard. In New Jersey, and to the south, through Maryland, 

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

 from South Carolina they spread westward along the Gulf border, 

 the inner limit of the region being about 100 miles from the Gulf 

 in Alabama, and 150 to 200 in Texas. Along the Mississippi River 

 the Gulf border region extends northward to southern Illinois. 



Unlike the Cretaceous formation, the marine tertiary strata do 

 not cover the Missouri region. Isolated brackish-water deposits 

 (determined to be such by the fossils) are found over parts of it, 

 and a great fresh-water Tertiary, sometimes called a Lignitic forma- 

 tion, spreads widely over the western plateaus or slopes of the 

 Rocky Mountains both north and south of the Upper Missouri (dis- 

 tantly-lined areas on the map, p. 133). 



Marine Tertiary beds occur also on the Pacific coast, in California 

 and Oregon. 



The Tertiary strata, as implied above, are either of fresh-water, 

 brackish-water, or marine origin, and they often vary greatly in 

 character from mile to mile. Instead of great strata of almost 

 continental extent and uniformity, as in the Silurian, there is the 

 diversity which is found among the modern formations of the 

 coast. 



These modern formations, now in progress, should be studied 

 in order to an understanding of the Tertiary. In one spot there 

 are mud-beds, with oysters or other mollusks ; in another 

 region, great estuary flats ; a little higher, on the same coast 

 perhaps, accumulations of beach-sands with worn shells, changing 

 in character every few rods. In another region, coral lime- 

 stones are in progress, as off the Florida coast ; and on other 

 shores, coarse shell limestones. Still further to comprehend 

 the diversity in the deposits, it is necessary to remember that by 

 the throwing up or removal of embankments on coasts, or by 

 change of level, salt-water marshes or estuaries may become brack- 

 ish-water, or wholly fresh-water, and the reverse, — each change 



