Leaving the ocean, look now at this river in our neighborhood, and see the slight muddiness which seems to color its waters.  What from?  Only a little earth and sand carried off from the banks as it flowed,—­very unimportant and small in quantity, doubtless, just at this moment and just at this spot.  But what of that little going on week after week, and century after century, throughout the whole course of the river, and throughout the whole course of every river and rivulet in our whole country and in every other country.  A vast amount of material must every year be thus torn from the land and given to the ocean.  For the land’s loss here is the ocean’s gain.

And, strange to say, we shall find that this same ocean, so busily engaged with the help of its tributary rivers in pulling down land, is no less busily engaged with their help in building it up.

You have sometimes seen directions upon a vial of medicine to “shake” before taking the dose.  When you have so shaken the bottle the clear liquid grows thick; and if you let it stand for awhile the thickness goes off, and a fine grain-like or dust-like substance settles down at the bottom—­the settlement or sediment of the medicine.  The finer this sediment, the slower it is in settling.  If you were to keep the liquid in gentle motion, the fine sediment would not settle down at the bottom.  With coarser and heavier grains the motion would have to be quicker to keep them supported in the water.

Now it is just the same thing with our rivers and streams.  Running water can support and carry along sand and earth, which in still water would quickly sink to the bottom; and the more rapid the movement of the water, the greater is the weight it is able to bear.

This is plainly to be seen in the case of a mountain torrent.  As it foams fiercely through its rocky bed it bears along, not only mud and sand and gravel, but stones and even small rocks, grinding the latter roughly together till they are gradually worn away, first to rounded pebbles, then to sand, and finally to mud.  The material thus swept away by a stream, ground fine, and carried out to sea—­part being dropped by the way on the river-bed—­is called detritus, which simply means worn-out material.

[Illustration:  A MOUNTAIN TORRENT.]

The tremendous carrying-power of a mountain torrent can scarcely be realized by those who have not observed it for themselves.  I have seen a little mountain-stream swell in the course of a heavy thunderstorm to such a torrent, brown and turbid with earth torn from the mountainside, and sweeping resistlessly along in its career a shower of stones and rock-fragments.  That which happens thus occasionally with many streams is more or less the work all the year round of many more.