Page:Popular Science Monthly Volume 39.djvu/260

248 known, of the different countries, we get 200,000 kilometres as the total length of the coast-lines. Hence, the supposed loss of 1,500 cubic metres per kilometre per year would give 300,000,000 cubic metres, or three tenths of a cubic kilometre. Thus, while the running waters take away ten and a half kilometres, the sea does not remove one twentieth of that quantity. Even supposing I have underestimated the height of the coast-banks, and have not given enough importance to the annual waste, let the figures I have used as the base of my calculations be tripled, we still find the effect of sea action a mere fraction, hardly significant, of that which is produced by the silent wash of the rivers. We can say here, as in many other cases, that what does the most work is not that which makes the most noise.

We have, in addition to this, to consider the solvent action of continental waters. They partially dissolve all the rocks, aided, as they are in the action, by carbonic acid; and they come to the sea charged with a considerably larger proportion of matter in solution than one would at first be liable to suppose. According to the labors of the English, American, and International commissions, which have especially studied the composition of the waters of rivers, particularly of the Mississippi, Danube, and Thames, the quantity of solid matters brought down in solution from the continents is not less than five cubic kilometres a year. This, added to the matter carried down mechanically, gives about 15 cubic kilometres, or, including the results of marine action, 16 cubic kilometres. This, then, is about what the continental masses lose each year.

Let us consider this supposed uniform plateau standing up 700 metres above the level of the sea. By the operation of the circumstances of which I have spoken, 16 cubic kilometres are taken from this mass every year. The continental surfaces covering 146,000,000 square kilometres, we calculate that a waste of 16 cubic kilometres will remove, each year, a layer of a millimetre thick. The débris from this layer will settle on the bottom of the sea and assume the form of sedimentary deposits; they will take the place, then, of a corresponding quantity of water, in consequence of which the sea will rise to a certain extent. The ratio of the continental surface being to that of the seas about as 100 to 252, the total result will be a lowering of the height of the plateau of about of a millimetre every year.

As many times as this of a millimetre is contained in 700 metres, or 700,000 millimetres, so many years will be required to bring about the disappearance of the dry land. Make the calculation, supposing the present intensity in the phenomena of destruction to continue, and you will find that it will take 4,500,000 years to wear the surface of the earth entirely away. This may be