DISTURBANCES CLOSING THE REPTILIAN AGE. 503 



America, is yet ascertained to have lived through the interval into 

 the Mammalian age. Moreover, very many of the genera and some 

 large families of species abounding in the Chalk are afterwards un- 

 known, as has been already illustrated. 



2. In the distribution of the Cretaceous beds, as contrasted with those 

 of the succeeding age. — The Cretaceous seas covered the summits 

 of parts of the Eocky Mountains and Andes. These lofty ranges 

 have since been raised, and in part the elevation took place 

 before the epoch of the Tertiary, whose marine beds lie at their 

 base. Vast additions were thus made to the continents. From 

 similar evidence it is known that the Pyrenees and Carpathians 

 were raised into existence in the early Tertiary ; and, while the 

 Alps were in part of much later date, dislocations and elevations 

 in the French Alps {Mount Viso system of De Beaumont) and the 

 southwestern extremity of the Jura are traced to the middle or 

 close of the Cretaceous. The surface of the Chalk of England is 

 described as bearing marks of very extensive denudation, proving 

 its elevation above the ocean in which it was formed before the 

 Tertiary beds were deposited. 



The evidence thus far collected is sufficient to sustain the state- 

 ment that the epoch following the close of the Mesozoic era, like 

 that after the Palaeozoic, was one of revolution, and that the dis- 

 turbances ended in extensive additions to the dry land of the 

 globe. 



But there is no reason to believe that the revolution was the 

 result of an instantaneous movement. It was probably slow in 

 progress, like others that had preceded it, and may have occupied 

 a long age. Moreover, this era of disturbance was continued 

 through the Tertiary period, during which the Pyrenees, Alps, Apen- 

 nines, Himalayas, and other mountains reached nearly to their 

 present altitude above the level of the ocean, and the continents 

 attained in general their full extent. 



The relative positions of the Cretaceous beds and marine Ter- 

 tiary in North America (see map, p. 133) afford data for estimating 

 the change of level after the Mesozoic era on the North American 

 Continent. In this way we learn that on the Atlantic border the 

 change was slight, and in general there was no upward movement ; 

 for the Tertiary formation mostly covers the Cretaceous. On the 

 Gulf border in Alabama the rise could not have exceeded 100 feet ; 

 along the Mississippi, towards the mouth of the Ohio, it may have 

 been 275 feet ; and about the head-waters of the Mississippi, the 

 great central plateau of the continent, 1700 feet. 



West of the Mississippi, as already stated, the changes were 



