46 PLAINS RIVERS. 



crossing the crest of an eminence, whose slope may be so 

 gradual as to be entirely imperceptible but it is never- 

 theless sufficient to create a rapid currentin a stream. 



In vast plains regions, like that of the American 

 prairies and the pampas, bounded towards the west 

 by a giant chain of mountains, and to the east either 

 by a great river, such as the Mississippi, and its con- 

 tinuation the Missouri, or by the ocean: there are 

 generally two sets of water-courses one of these consists 

 of streams proceeding from some central ridge in the 

 continent, which make their way either to the great 

 river or to the sea ; whilst the other, mostly consisting 

 of mountain torrents, in many instances become lost 

 in the plains. It may be that these latter merely grow 

 smaller and smaller, until at length they disappear 

 altogether; or else, they end in salt lakes or marshes, 

 such as we have already described. This of course is 

 more particularly the case towards the warmer and 

 drier limits of the Region of the Great Plains, which 

 adjoin the Desert Zone, and nearly all plains countries 

 and extensive elevated table lands furnish numerous 

 examples of such streams. In Europe we may refer 

 to the case of the Guadarama River, in Spain, which 

 is lost in the plains and marshes of Murcia a locality 

 visited by few travellers, but which, we can say from 

 personal observation, is quite worth a visit, as a mat- 

 ter of geographical interest. In South America we have 

 also seen great numbers of similar cases. 



As a rule, along the banks, and in the meadows 

 subject to floods, which lie along the margins of the 

 rivers and larger streams, more or less timber is to be 

 found; heavy timber is however rare, and in most 

 cases it consists either of medium-sized deciduous trees, 



