FRESH-WATER STREAMS. 



641 



2. Mechanical Effects of Rivers. 



The mechanical effects of fresh waters are, — 



1. Erosion, or wear. 



2. Transportation of earth, gravel, stones, etc. 



3. Distribution of transported material, and the formation of frag- 

 mental deposits. 



1. Erosion. 



1. General Statement of the Effects of Erosion. — The effects of 

 erosion are seen,frst, in the imprint of the falling rain-drop, — a tri- 

 fling matter to most eyes, but not so to the geologist ; for it remains, 

 among the records of the earliest and latest strata, to show that it 

 rained then as now, and to teach us where the lauds at the time lay 

 above the ocean. It is, therefore, a part of the markings in which the 

 geographical history of the globe is registered. 



Second. The gathering drops make the rill, and the rill its little 

 furrow ; rills combine into rivulets, and rivulets make a gully down 

 the hill-side; rivulets unite to form torrents, and these work with accu- 

 mulating force, and excavate deep gorges in the declivities. Other 

 torrents form in the same manner about the mountain-ridge, and 

 pursue the same work of erosion, until the slopes are a series of val- 

 leys and ridges, and the summit a bold crest, overlooking the eroding 

 waters. 



2. Progress of erosion in the Formation of Valleys or River Courses. 

 — The mist and rains or snow about the higher parts of mountains 

 are the main source of the water. As the first-made streamlets gather 

 into larger streams, on the descent, and are largest below, there the 

 valley first takes shape. 



Fig. 1076. 



Fig. 1077. 



Let A B (Fig. 1076) represent a profile of a declivity. As the ero- 

 sion goes on, a valley is formed along I m, on the principle just stated, 

 with but little channelling above along A I. On reaching m, the most 

 of the descent of the declivity is made : the waters from m to B have, 

 therefore, but little eroding power at bottom, and they commence to 

 erode laterally during freshets, undermining the cliffs on either side, 

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