6 PHYSICAL GEOGRAPHY OF 



Lakes and the Gulf of Mexico, and extends from the Atlantic to the Rocky 

 mountains. 



From the summit of these mountains a great plain slopes gently to the east, 

 along which flow all the streams that enter the lower Mississippi and the Gulf 

 of Mexico, from the west. Another plain, of nearly equal extent, and equally 

 gentle in its inclination, descends from the north, along which flow the northern 

 tributaries of the Ohio and the Mississippi itself, until it unites with the great 

 Missouri, flowing along the irregular line which marks the intersection of 

 these vast surfaces ; while another plain, descending from the summit of the 

 Alleghany range, conveys the waters of the Cumberland and Tennessee, and 

 all the southern tributaries of the Ohio, and intersects the great plain from the 

 north, in the valley of the Ohio, and the greater plain from the west, in the val- 

 ley of the lower Mississippi. 



The intersection of the great slopes from the south and east with those from 

 the north and west, near the confluence of the Mississippi, Missouri, and Ohio, 

 creates what deserves to be regarded as a geographical centre of this remark- 

 able region — a position which is rapidly becoming, from causes depending upon 

 its physical geography almost entirely, the centre of commerce, wealth, and 

 population, of the whole North American Continent. 



A single plain spreads from the summit of the Alleghany mountains to the 

 Atlantic coast, and turning the northern and southern flanks of that range, runs 

 into the slope which sustains the southern tributaries of the Ohio. 



The word plain is adopted here for the convenience of description only, and 

 is not to be received in a literal sense. These great surfaces are furrowed by 

 valleys, and relieved in places by hills and even mountains ; yet these mountains 

 are of inconsiderable extent compared with the vast area of the region described, 

 and rest upon the great slopes which descend from the dividing ranges. 



The Alleghany mountain is thus an elevated centre, from which the land 

 descends and the waters flow in all directions. Where it approaches the coast 

 most nearly, in Pennsylvania and Virginia, the fall is rapid and the rivers of 

 quick descent. They are therefore difficult of navigation ; but as their develop- 

 ment is short, they are less needed for navigation. 



On the reverse, or western side of this elevated range, the sources of the 

 streams and the land which their waters irrigate and fertilize are far from the 

 ocean, the great recipient of their commerce. And nature has here provided 

 for the future necessities of an enterprising and improving race, by giving the 

 streams which are to float the products of toil a greater space to glide over, a 

 longer development, and a descent just sufficient to carry off the surplus water, 

 with a current so gentle that it may be stemmed by the power which the 

 genius of man is capable of calling into operation. 



This law prevails thi'oughout the globe. Those countries which are far from 

 the ocean have long and gentle rivers, serving to drain oflf their surplus water, 

 and to bear their surplus products. The sides of the mountains which front 

 towards the ocean are abrupt and steep, their valleys are narrow and precipi- 



