Masterpieces of Science 



bench of detrital matter. In fact, all this mar- 

 ginal accumulation is of temporary duration, 

 for the stream is as yet wild, and in its annual 

 floods is apt to undo the construction- work of the 

 previous years. 



When the stream comes to have a distinct and 

 somewhat enduring alluvial belt on either side 

 of its path, it has entered on the stage of a river. 

 It is indeed on the presence of this marginal 

 accumulation that we most rest the distinction 

 between a torrent and a river. From the place 

 where the terraces begin to form, downward to 

 the mouth of the stream, the conditions of its 

 flow are vastly affected by its reactions upon 

 this detrital matter. In most cases, with each 

 mile of its descent the magnitude of these de- 

 posits increases. The alluvial lands stretch 

 farther and farther on either side; the materials 

 which compose them grow finer as we descend 

 in the valley, for the reason that with this de- 

 scent the slope of the stream in most cases 

 steadfastly diminishes and its ability to urge 

 forward coarse sediments decreases in a rapid 

 ratio. 



The alluvial deposits which border our rivers 

 owe their existence to the fact that the torrential 

 head- waters, by their great velocity, bear for- 

 ward, beyond the mountain districts, a large 

 amount of materials which are of such a coarse 

 nature that the larger but less powerful lower 

 part of the stream cannot urge them onward to 

 the sea. In all its journey to the ocean, the 

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