“Here we have a definite figure, expressing the actual yearly continental erosion, showing that, if only this erosion were to operate, the entire mass of unsubmerged land would disappear in less than 10,000,000 years.
“But rain and rivers are not the only agencies; there are other factors which contribute to the gradual destruction of the dry land:
“First, there is the erosion of the sea. It is impossible to select a better example of this than the Britannic isles; for they are exposed, by their situation, to the onslaught of the Atlantic, whose billows, driven by the prevailing southwest wind, meet with no obstacle to their progress. Now, the average recession of the English coast is certainly less than three meters per century. Let us apply this rate to the seacoasts of the world, and see what will happen.
“We may proceed in two ways: First, we may estimate the loss in volume for the entire coastline of the world, on the basis of three centimeters per year. To do this, we should have to know the length of the shoreline and the mean height of the coast. The former is about 200,000 kilometers. As to the present average height of the coasts above the sea, 100 meters would certainly be a liberal estimate. Hence, a recession of three centimeters corresponds to an annual loss of three cubic meters per running meter, or, for the 200,000 kilometers of coastline, 600,000,000 cubic meters, which is only six-tenths of a cubic kilometer. In other words, the erosion due to the sea would only amount to one-seventeenth that of the rivers.
“It may perhaps be objected, that, as the altitude actually increases from the coastline toward the interior, the same rate of recession would, in time, involve a greater loss in volume. Is this objection well founded? No; for the tendency of the rain and watercourses being, as we have said, to lower the surface-level, this action would keep pace with that of the sea.
“Again, the area of the dry land being 145,000,000 square kilometers, a circle of equal area would have a radius of 6800 kilometers. But the circumference of this circle would be only 40,000 kilometers; that is to say, the sea could exercise upon the circle but one-fifth the erosive action which it actually does upon the indented outline of our shores. We may, therefore, admit that the erosive action of the sea upon the dry land is five times greater than it would be upon an equivalent circular area. Certainly this estimate is a maximum; for it is logical to suppose that, when the narrow peninsulas have been eaten away by the sea, the ratio of the perimeter to the surface will decrease more and more—that is, the action of the sea will be less effective. In any event, since, at the rate of three centimeters per year, a radius of 6800 kilometers would disappear in 226,600,000 years, one-fifth of this interval, or about 45,000,000 years, would represent the minimum time necessary for the destruction of the land by the sea; this would correspond to an intensity of action scarcely more than one-fifth that of the rivers and rain.
“Taken together, these mechanical causes would, therefore, involve every year a loss in volume of twelve cubic kilometers, which, for a total of 100,000,000, would bring about the complete submergence of the dry land in a little more than 8,000,000 years.
“But we are far from having exhausted our analysis of the phenomena in question. Water is not only a mechanical agent; it is also a powerful dissolvent, far more powerful than we might suppose, because of the large amount of carbonic acid which it absorbs either from the atmosphere or from the decomposed organic matter of the soil. All subterranean waters become charged with substances which it has thus chemically abstracted from the minerals of the rocks through which it percolates.
“River water contains, per cubic kilometer, about 182 tons of matter in solution. The rivers of the world bring yearly to the sea, nearly five cubic kilometers of such matter. The annual loss to the dry land, therefore, from these various causes, is seventeen instead of twelve cubic kilometers; so that the total of 100,000,000 would disappear, not in eight, but in a little less than six million years.
“This figure must be still further modified. For we must not forget that the sediment thus brought to the sea and displacing a certain amount of water, will cause a rise of the sea-level, accelerating by just so much the levelling process due to the wearing away of the continents.
“It is easy to estimate the effect of this new factor. Indeed, for a given thickness lost by the plateau heretofore assumed, the sea-level must rise by an amount corresponding to the volume of the submarine deposit, which must exactly equal that of the sediment brought down. Calculation shows that, in round numbers, the loss in volume will be twenty-four cubic kilometers.
“Having accounted for an annual loss of twenty-four cubic kilometers, are we now in a position to conclude what time will be necessary for the complete disappearance of the dry land, always supposing the indefinite continuance of present conditions?
“Certainly, gentlemen; for, after examining the objection which might be made apropos of volcanic eruptions, we find that the latter aid rather than retard the disintegrating process.
“We believe, therefore, that we may fearlessly accept the above estimate of twenty-four cubic kilometers, as a basis of calculation; and as this figure is contained 4,166,666 times in 100,000,000, which represents the volume of the continents, we are authorized to infer that under the sole action of forces now in operation, provided no other movements of the soil occur, the dry land will totally disappear within a period of
