RECORDS LEFT BY RIVERS 



If now, says he, we assume that all over the world this 

 is the amount carried down, we can see how seriously the 

 level of the land is lowered by rivers. The Mississippi 

 carries from the land it drains every year the innnrth part 

 of a foot of rock. If we take the general height of the 

 land of the whole globe to be 2100 feet, and suppose it to be 

 continuously wasted at this rate, then the whole dry land 

 would be carried into the sea in 12,600,000 years. Or if 

 we assume the average height of the continent of Europe 

 to be 940 feet, and to be lowered by its rivers at the 

 same rate, then the last vestige of Europe would have dis- 

 appeared in 8,640,000 years. Such figures are of course 

 not exact; and it must always be remembered that 

 the rivers are merely robbing Peter to pay Paul, and 

 whatever they take away are always putting somewhere 

 else, but we may learn from the foregoing considerations 

 that the lowering of the land is much more rapid than is 

 sometimes supposed. Another thing about the excavating 

 work of rivers has to be remembered. The torrents 

 carry sand, shingle, and rock with them, and these very 

 materials act as agents of destruction on the beds of the 

 watercourses. If we want to polish brass or steel we mix 

 emery powder (or something finer or coarser) with the 

 polishing liquid. The torrent or river uses sand or 

 shingle as its polishing powder. It then wears out the 

 rock over which it travels, and sometimes carves it 

 into holes or caverns, gorges or ravines. Sometimes the 

 process is varied, as when a stream finds its way over a hard 

 rock which overlies a softer rock. If the arrangement is 

 like that of a series of steps (there may be only one or two 



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