Having learned how the three agents of denudation—namely, rain, rivers, and glaciers—accomplish their work, let us now take a wider view of the subject and consider the results of their united efforts both in the present and in the past.
We have already alluded to the enormous amount of solid matter brought down to the sea every year by rivers (see chap. v., pp. [166]-[168]), and we pointed out that all this represents so much débris swept off the land through which the rivers flow; also that it comes down in three ways, one part being suspended in the water as fine mud, another part being pushed along the river-bed as gravel, etc., while a third part is the carbonate of lime and other mineral matter in a dissolved state, and therefore invisible.
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.
Now, it is quite clear that all this débris must have come from the immense area that is drained by the Mississippi. It could not have been supplied by any rivers except those that are its tributaries. And so if we can find out what is the extent of this area, it is not difficult to calculate how much its general surface must have been lowered, or in other words, how much must have been worn away from it in order to supply all the material. This area is reckoned at 1,147,000 square miles; and a very simple calculation tells us that the general surface would thus be lowered to the extent of 1/6000th part of a foot. That of course means that one foot would be worn away in six thousand years. On high ground and among mountains the rate of denudation would of course be much greater; but we are now dealing with an average for the whole surface.
The next thing we require to finish this calculation is the average or mean height of the American continent. This was reckoned by the celebrated Humboldt at 748 feet. Now if we may assume that all this continent is being worn down at the same rate of one foot in six thousand years (which is a reasonable assumption), we find, by a simple process of multiplication, that it would require about four and a half millions of years for rain and rivers to wash it all away until its surface was all at the sea-level (with perhaps a few little islands projecting here and there as relics of its vast denudation). This is a very interesting result; and if the above measurements are reliable, they afford us some idea of the rate at which denudation takes place at the present time.
By a similar process it has been calculated the British Isles might be levelled in about five and a half millions of years. Geologists do not pretend to have solved this problem accurately; that is impossible with our present knowledge. But even as rough estimates these results are very valuable, especially when we come to study the structure of the land in different countries, and to find out therefrom, by actual measurement, how much solid rock has been removed. We will now give some examples of this; but perhaps a simple illustration will make our meaning clearer.
Suppose we picked up an old pair of boots, and found the soles worn away in the centre. It would be easy to find out how much had been worn away over the holes by simply measuring the thickness of leather at the sides, where we will suppose that they were protected by strong nails. Geologists apply a very similar kind of method in order to find out how much rock has been removed from a certain region of the earth. One of the simplest cases of this kind is that of the area known as the Weald of Kent, Surrey, and Sussex (see illustration, Fig. [1]). A great deal of denudation has taken place here, because there is ample evidence to prove that the great "formation" known as the Chalk (now seen in the North and South Downs) once stretched right across; and below this came the lower greensand and Weald clay. They spread over this area in a low arch of which we now only see the ruins.