MATTER IN SUSPENSION. 57 



We find, moreover, as a matter of fact, that, while 

 masses of rocks miles in thickness and thousands of miles 

 in extent whole continents, indeed have been formed in 

 the sea, the fresh-water deposits of rivers, lakes, and estuaries, 

 are to be reckoned only by some thousands of feet. 



The Rhine, as we have seen, drops part of its load in 

 the comparatively still waters of Lake Constance, but 

 though it emerges thence no longer muddy, its dark green 

 colour shows that it is by no means perfectly filtered, and 

 the finer particles are carried farther still. 



But besides the mud and those still finer particles which 

 give to water its green or blue tint, rivers carry away a vast 

 amount of mineral matter, which, though absolutely invisible 

 to us, is none the less important A very small pinch of 

 powdered chalk will, we know, make a large glass of water 

 quite milky, while a handful of salt will disappear, leaving it 

 just as clear as before. The chalk is held in suspension, and 

 in time will settle at the bottom of the glass, while the salt, 

 being dissolved, or held in solution, will not reappear until 

 the water is removed, which may be done either by leaving 

 it to evaporate or by boiling. When sea-water is boiled its 

 various salts are left behind in the form of crystals, and the 

 steam arising from it, if caught and condensed into water, 

 will be found to be almost pure. In like manner the sun 

 draws up large quantities of almost pure water from the 

 ocean, leaving the salts behind. 



It is very evident that rivers are most heavily laden with 

 sediment either during the rainy season, when large quanti- 

 ties of mineral matter, loosened by various agents, are washed 

 down from their banks, or when the snow is melting on the 



