TERTIARY PERIOD. 531 



vation of the mountain-region westward, the Missouri and other 

 streams rising in the mountains had begun to exist-. 



Yet the elevation of 6000 or 7000 feet, which began after the 

 Cretaceous, was but partly completed ; the rivers were therefore 

 smaller than now, and the region, as Hayden rightly infers from 

 the great fresh-water Tertiary deposits, was covered by one or more 

 vast fresh-water lakes. The transition from the salt-water sea of 

 the Cretaceous to the emerged condition of the land in the Tertiary 

 is marked in the brackish-water deposits which lie at the base of 

 the series. 



After the close of the Vicksburg epoch, referred to the Upper 

 Eocene, there appears to have been a further reduction of the 

 Mexican Gulf; for the Miocene and Pliocene beds are not recog- 

 nized on its borders, unless very near the sea. 



The Atlantic Tertiary region must have remained submerged 

 until after the Miocene era. The absence from most parts of the 

 coast of deposits that can properly be identified as Pliocene is a 

 remarkable fact, and seems to show that the continent during the 

 Pliocene era had at least its present breadth along the larger part 

 of the Atlantic coast, if not a still greater eastward extension. 



The change of water-level causing this enlargement of the area 

 of dry land was probably not confined to the border of the conti- 

 nent, but was part of a general change in which a large part of the 

 continent partook, especially the Eocky Mountain regions of the 

 West, and sparingly the country south of the Appalachian region, 

 or northwest of Florida. As to the latter, there is definite proof of 

 elevation in the present height of the Tertiary in parts of Georgia 

 and Alabama ; for while in general the beds on the Gulf border 

 are but 100 to 200 feet above the sea, near Milledgeville, Georgia, 

 they are now 600 feet, and near Montgomery about 800 feet. The 

 position of the region, in a line with the general trend of Florida, 

 suggests that its elevation may have been connected with that of 

 the Peninsula of Florida itself. Moreover, the northwestward trend 

 corresponds with that of the Rocky Mountain region, which was in 

 process of elevation through the Tertiary, or after the Mesozoic, 

 and not with that of the Appalachian chain, which was raised 

 mainly soon after the Palaeozoic. 



With regard to the Rocky Mountain region, the great thickness 

 over it of the Miocene and Pliocene shows a prolonged continua- 

 tion of the lacustrine condition of the great area, and renders it 

 altogether probable that the mountains did not attain their full 

 altitude until late in the Tertiary period. The deposition of the 

 Lignite formation of the Eocene appears to have been followed by 



