MISSOURI AND MISSISSIPPI. 



131. It must not be inferred from what has been here stated 

 that the valley of every river is formed by slopes, having 

 declivities obvious to the eye, or limited by chains of mountains 

 of conspicuous elevation. Most commonly it is quite otherwise, 

 the declivities of the valley being so gentle as to be almost 

 imperceptible, and the summits of the ridges, which limit it, 

 having no elevation which entitles them to the name of mountains. 



132. Portage. Where the navigation of a river is impeded by 

 waterfalls, rapids, shallows or other natural obstructions, the 

 space over which goods, and sometimes canoes or boats have to be 

 carried, to meet the navigable part of the stream again, is called 

 a portage. 



133. Examples of rivers of Worth America. This division 

 of the western continent being traversed by two ridges, the Rocky 

 Mountains and the Alleghanies, whose general directions are nearly 

 parallel, is divided into three zones, running north and south, the 

 centre and broadest of which is included between the two ridges, 

 the eastern zone sloping down from the Alleghanies to the Atlantic, 

 and the western from the Rocky Mountains to the Pacific. 



134. Eastern rivers. The disposition of the general relief 

 of the continent, shown by a section of it, running east and west, 

 determines three different directions for the rivers. Those which 

 are formed of the drainage of the eastern slopes of the Alleghanies, 

 now eastward into the Atlantic, and the distance of the ridge of 

 the Alleghanies from the Atlantic coast not being great, and the 

 surface of the intervening zone being nearly plane, the lengths of 

 the rivers are inconsiderable, and their streams not rapid. 



135. "Western rivers. In the same manner the drainage of 

 the western slope of the Rocky Mountains forms a series of rivers 

 which flowing westward fall into the Pacific. 



136. The Mississippi and its tributaries. The great ex- 

 tent of the valley included between the ridges of the Alleghanies 

 and the Rocky Mountains, and its great capacity for cultivation, 

 confer upon it an importance which is immensely augmented by 

 the great length through which the rivers which traverse it, are 

 navigable. 



137. The drainage of the western slopes of the Alleghanies, and 

 that of the eastern slopes of the Rocky Mountains, form two 

 systems of rivers, the one flowing from west to east, and the other 

 from east to west, and meeting in a common bed in the centre of 

 the valley. They thus form a main stream traversing the valley 

 from north to south, the magnitude of which increases as it 

 descends southwards, in proportion to the number and magnitude 

 of the streams which flow into it from the one side or the other. 

 This central stream is the Mississippi, which gives its name 



159 



