STREAMS 



73 



dition of the rocks, the metallic substances are so scattered 

 that they cannot be profitably extracted. 



Naturally, the veins themselves are not composed of one 

 substance alone, because several different precipitates may 

 be formed. But there is a 

 decided grouping of valu- 

 able metals, and these can 

 then be readily separated 

 by means of electricity. 



67. Streams. Streams 

 usually carry mud and sand 

 along with them ; this is 

 particularly well seen after 

 a storm when rivers and 

 brooks are muddy. The 

 puddles which collect at 

 the foot of a hill after a 

 storm are muddy because 

 of the particles of soil 

 gathered by the water as 

 it runs down the hill. The 

 particles- are not dissolved 

 in the water, but are held 

 there in suspension, as we 

 call it technically. The river made muddy after a storm by 

 suspended particles usually becomes clear and transparent 

 after it has traveled onward for miles, because, as it travels, 

 the particles drop to the bottom and aw* deposited there. 

 Hence, materials suspended in the water are borne along 

 and deposited at various places (Fig. 34). The amount of 

 deposition by large rivers is so great that in some places 

 channels fill up and must be dredged annually, and vessels are 

 sometimes caught in the deposit and have to be towed away. 



FIG. 33. Mineral matter precipitated fron 

 solution is deposited in crevices and forms 

 veins. 



