Now, it is quite plain that rain and rivers, in sweeping away so much solid matter from the surface of the land, must tend in the course of time to lower its general level; and it therefore seems to follow that after the lapse of ages any given continent or large island might be entirely washed away, or in other words, reduced to the level of the sea. This would certainly happen were it not that the lands of the world seem to be slowly rising, so that the denudation going on at the surface appears to be counterbalanced by continued upheaval. But, supposing no upheaval took place, how long would it take for rain and rivers to wear away a whole continent? Let us see if there is any way of answering this difficult question, for if it can be even partially solved, it will help us to realise the enormous length of time that must have been required to bring about the results of denudation that we see all around us. Although the calculations that have been made on this subject are very complicated, yet the principle on which they are based is quite simple. For an answer to our question we must go to the rivers again, and measure the work they do in transporting solid matter down to the sea. Let us take the Mississippi as a typical big river, for it has been more carefully studied than any other, and it drains a very extensive area, embracing many varieties of climate, rock, and soil. As the result of many observations carried on continuously at different parts of the river for months together, the engineers who conducted the investigation found that the annual discharge of water by this river is about nineteen thousand millions of cubic feet, and that on the average the amount of sediment it contains is about a 1/1500th part by weight. But besides the matter in suspension, they observed that a large amount of sand, gravel, and stones is being constantly pushed along the bottom of the river. This they estimated at over seven hundred and fifty millions of cubic feet. They also calculated that the Mississippi brings down every year more than eight hundred thousand million pounds of mud. Putting the two together, they found (as before stated) that the amount of solid matter thus transported down to the Gulf of Mexico may be represented by a layer 268 feet high, covering a space of one square mile; that is, without allowing for what is brought down dissolved in the water, which may be neglected in order to prevent any exaggeration.