The aspect of its force with which we are, however, most concerned is that which is directed to lowering gradually the surface of the land. In the last chapter we showed how much mineral might be dissolved in the waters of rivers. If we are to include also the amount of mud, sand, and other things classed altogether as silt which a river carries down, the figures become much more imposing. Sir Archibald Geikie says that, taking the Mississippi as a typical river (it is as good an example as would be found, because in its great length it passes through many different kinds of land, soil, and climate), we may assume that the average amount of sediment carried down by a river is one part of sediment to every 1500 parts of water.
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 1/6000th 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 disappeared 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 water-courses. 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 steps) it is possible for the river as it foams in a waterfall over the hard step at the top to eat its way into the lower softer step. The lower softer step will gradually disappear, and then the waterfall, still eating its way in, will begin to undermine the hard top step, and when that has gone on long enough the hard top step will fall down and the waterfall will have to begin a little farther up the stream. In this way a waterfall, gorge, or ravine can be constructed by a river.