MOUNTAIN RTANGES. 



209 



tern is composed of rocks of the transition class, or 

 the oldest formations of the non-fossiliferous sec- 

 ondary group. The Appalachian belt consists of 

 secondary rocks of the oldest kind, containing fossils, 

 but no rocks as recent, apparently, as the bitumi- 

 nous coal series. The Alleghany range is composed 

 also of secondary formations, which seem to owe 

 their configuration, which is that of vast piles of 

 nearly horizontal strata, rising from a plain inter- 

 sected by innumerable deep valleys of denudation, 

 to causes which seem to have removed part of the 

 high plateau on which they stand, rather than to di- 

 rect uplifting forces, such as appear to have eleva- 

 ted the more irregular and convulsed systems of the 

 other three mountain systems. 



Thus nearly the whole of North America, is di- 

 vided into two immense plains, the one lying be- 

 tween the Atlantic and the Appalachian or Allegha- 

 ny Mountains, and extending from Long Island to 

 tlie Gulf of Mexico ; the other, which may be called 

 the central basin of the continent, from the Alle- 

 ghanies to the Rocky Mountains, and expanding 

 from the Gulf of Mexico, widening as it stretches 

 northward until it reaches the Arctic Sea and Hud- 

 son's Bay. The first great plain, extending from 

 the Atlantic to the adjacent mountains, is, in the 

 Southern states, nearly 200 miles in breadth, and 

 throughout its whole extent it is separated into two 

 tracts, strongly contrasted with each other both as 

 respects .their geographical and geological features. 

 The boundary which divides them is the eastern 

 edge of a low, undulating line of primary rocks, 

 which, forming the termination of the upper or 

 rocky tract, separates it from the lower flat and 

 sandy plains, with every appearance of having beea 

 at one time the line of the coast. 



From New-Jersey to North Carolina, this bound- 

 ary, beginning the rocky country, presents a well- 

 marked barrier to the tide in nearly all the rivers 

 R 



