

AQUEOUS AGENCIES. 27 



thus easily understand the prodigious power of mountain- 

 torrents when swollen by heavy rains. 



It follows from the above that., if a stream be carrying 

 all it can, the least checking of its velocity will cause 

 abundant deposit, and the least increase of its velocity 

 will cause it to take up again what it had previously 

 deposited i. e., it will scour its bed and banks. 



Sorting- Power of Water. If we take a- handful of 

 earth and throw it into a deep basin, and, after allowing 

 it to settle, pour off the water and examine the sediment, 

 we shall find that it is neatly sorted, the coarser particles 

 being at the bottom, and above this finer and finer, until 

 a very fine, smooth mud forms the top. The earth will 

 be still better sorted if we throw it into running water. 

 In this case the coarser will drop first, i. e., higher up, 

 and the finer lower and lower, until only the finest will be 

 carried far down the stream. This is especially the case 

 if the velocity decreases as we go down-stream, as is 

 usually the case in natural streams. Thus, pebbles are 

 found in torrent-beds, and fine mud in lower parts of 

 streams. 



Stratification. If we examine carefully the mud or 

 sand of a river-bank or lake-margin, we sliall always find 

 them stratified, i. e., in layers of slightly different color 

 and grain. This is easily explained by the sorting power 

 of water. If the water be still, as in a lake or pond, then 

 with every rain earth is brought in, and by settling is 

 sorted, the finest falling last. Thus the coarse material 

 of one rain falls on the fine of the previous rain, and 

 every rain is marked by a separate layer. In rivers, the 

 same result follows, but the explanation is a little differ- 

 ent. The velocity of the current is changing from day 

 to day on account of the varying supply of water. The 

 stream -lines also are continually shifting from side to 

 side. Thus the velocity at any one point is all the time 

 changing, and therefore the character of the material 



