“The dry land, if spread out in a layer of uniform thickness, would constitute a plateau of about 700 meters altitude above the sea-level. Admitting that its total area is 145,000,000 square kilometers, it follows that its volume is about 101,500,000, or, in round numbers, 100,000,000 cubic kilometers. Such is the large, yet definite mass, with which the external agencies of destruction must contend. [Illustration: THE RIVERS CEASE TO FLOW. ] “Taken together, the rivers of the world may be considered as emptying, every year, into the sea 23,000 cubic kilometers of water (in other words, 23,000 milliards of cubic meters). This would give a volume of solid matter carried yearly to the sea, equal to 10.43 cubic kilometers, if we accept the established ratio of thirty-eight parts of suspended material in 100,000 parts of water. The ratio of this amount to the total volume of the dry land is one to 9,730,000. If the dry land were a level plateau of 700 meters altitude, it would lose, by fluid erosion alone, a slice of about _seven one-hundredths of a millimeter in thickness yearly_, or one millimeter every fourteen years—say _seven millimeters per century_. “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 sea-coasts 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 coast-line of the world, on the basis of three centimeters per year. To do this, we should have to know the length of the shore-line 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 coast-line, 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.