RECORDS LEFT BY RIVERS 



a manufactory. It comes down several thousand feet. In 

 the manufactory there is a vent-hole, out of which the 

 water can be allowed to shoot. The vent-hole is about 

 an inch in diameter ; and the water rushes out with such 

 swiftness and force that the water-jet becomes as rigid as 

 steel. It is impossible to cut through this water-jet ; and 

 if any one were to try to do so with a sword, the sword 

 might break but it could never pierce or pass through 

 those swiftly moving particles of water. A more common- 

 place illustration is the use that is sometimes made of 

 water-jets to break up the surfaces of rock in quarries ; 

 nor must it be forgotten that horse-power of great value 

 and extent for electric lighting and other purposes is 

 always being drawn from waterfalls. Thus as a mechanical 

 force merely the river can be immensely powerful ; and 

 must leave marks of its power on the rocks. 



The aspect of its force with which we are, however, 

 most concerned is that which is directed to lowering 

 gradually the surface of the land. In the last chapter we 

 showed how much mineral might be dissolved in the 

 waters of rivers. If we are to include also the amount of 

 mud, sand, and other things classed altogether as silt 

 which a river carries down, the figures become much more 

 imposing. Sir Archibald Geikie says that, taking the 

 Mississippi as a typical river (it is as good an example as 

 would be found, because in its great length it passes 

 through many different kinds of land, soil, and climate), 

 we may assume that the average amount of sediment 

 carried down by a river is one part of sediment to every 

 1500 parts of water. 



