RIVERS. 635 



4. Force of running water. — According to Hopkins, the force of run- 

 ning water varies as the sixth power of the velocity : so that doubling 

 the rate increases sixty-four times the force. If a stream running 

 ten miles an hour would just move a block of five tons' weight, 

 then a current of fifteen miles would move a similar block of fifty- 

 five tons ; one of twenty miles, a block of three hundred and twenty 

 tons ; while a current of two miles an hour, or three feet per second, 

 would move a pebble of similar form only a few ounces in weight ; 

 at one foot per second, gravel ; at six inches, fine sand ; at three 

 inches, fine clay. 



Other characteristics of rivers are brought out in the following 

 pages. 



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 

 fragmental deposits. 



1. Erosion. 



1. General statement of the effects of erosion. — The effects of erosion 

 are seen, first, in the imprint of the falling rain-drop, — a trifling 

 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 lands 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 accumulating force, and excavate deep gorges in the declivi- 

 ties. 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 valleys and ridges, and the summit a bold crest overlook- 

 ing the eroding waters. 



2. Progress of erosion in the formation of valleys or river-courses. — The 

 mist and rains about the higher parts of mountains are usually the 

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

 ing into larger streams through the course of the descent, and are 

 largest below, the torrent has its greatest force towards the bottom 

 of the declivity, and there the valley first takes shape and size. 



Let A B (fig. 938) represent a profile of a declivity. As the ero- 



