DISSOLVED MATERIALS 101 



as the sixth power of the velocity. If the rapidity of a stream be . 

 doubled, it can carry 64 times as much as before. The destruc- 

 tiveness of sudden and violent floods is thus explained. In the 

 terrible flood which overwhelmed Johnstown, Pennsylvania, in 

 1889, great locomotives and massive iron bridges were swept off, 

 it is hardly an exaggeration to say, like straws, and huge boulders 

 carried along like pebbles. 



It obviously follows from the relation obtaining between velocity 

 and transporting power, that a slight increase in the rapidity of a 

 stream will largely augment the load which it carries, provided the 

 stream obtains as much material as it can carry, while a slight 

 reduction of velocity will cause the deposition of a large part of 

 that load. The buoyancy of water adds, in an important degree, 

 to its ability to sweep along sediment, because when any substance 

 is immersed in water, it loses weight to an amount equal to the 

 weight of an equal bulk of water. The specific gravity of most 

 rocks is from two and one-half to three, so that when immersed 

 they lose from one-third to two-fifths of their weight in air. The 

 shape of the fragments is likewise a factor in determining the 

 velocity requisite to move them ; the larger the surface of the 

 fragment in proportion to its weight, the more easily it is carried 

 in suspension. Thus flat grains or scales are carried farther than 

 round ones ; while, on the other hand, rounded fragments are more 

 easily rolled along the bottom, when too heavy for the current 

 to lift. 



The greater part of the debris or sediment which a stream car- 

 ries is furnished to it by the destructive activity of the atmosphere ; 

 the rains wash in the finer materials, while frost and landslips bring 

 in the larger masses which are carried down by mountain torrents. 

 To this material the river adds that which is derived from its own 

 work in the cutting away of its banks and bed. 



Materials in Solution. — In addition to what the river carries 

 down mechanically in suspension or sweeps along the bottom, 

 there is a third class of material ; namely, that which is dissolved 

 in the waters of the stream. Dissolved matters are always present 

 in greater or less quantity, and are the same in kind as those 



