THE FRIENDLY SOIL 
and taken their toll of the grist they have ground. 
Sometimes these mills take the whole grist and leave 
the rocks bare; but usually they leave a residuum 
in which life strikes its roots. We do not see all that 
the waters take from the soil. They have invisible 
pockets in which they carry away all the more 
soluble parts, such as lime, soda, potash, silica, 
magnesia, and others, and leave for the land the more 
insoluble parts. These, too, in times of flood they 
carry away in suspension, in the shape of sand, silt, 
mud, gravel, and the like. When the waters really 
digest the rocks, they hold the various minerals in 
solution, and run limpid and dancing to the sea; 
when they have an undigested burden, they run 
angry and turbid. 
It is estimated that the Hudson River deposits 
in the sea each year four hundred and forty 
thousand tons of mineral matter in solution which 
it has taken from the land, and the Mississippi one 
hundred and twelve million tons. Each carries 
away about four times as much in suspension. The 
digestive or chemical power of water, then, is only 
about one fourth as great as its mechanical power. 
Between the two the land is made to pay heavy toll 
to the sea. But in time, in geologic time, it all comes 
back. The suspended particles are dropped and go 
to make up the sedimentary rocks, while the solutes 
help cement the material of these rocks together, 
and also nourish the sea life from which limestone 
169 
