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.