Page:Popular Science Monthly Volume 39.djvu/259

Rh pares away the continental masses. Mr. J. Murray, of Scotland, from the study of all that has been published on this subject, of which he has himself furnished a considerable proportion, has found that the outflow of the nineteen principal rivers of the earth is 3,610 cubic kilometres a year. These 3,610 cubic kilometres bring to the sea a mass of solid matter in suspension equivalent to one cubic kilometre and making a proportion in value of 38 parts per 100,000. On the other hand, meteorological observations have become precise enough to enable us to estimate approximately the annual outflow of all the rivers of the earth. Mr. Murray puts it at 23,000 cubic kilometres. Applying to this figure the same proportion of 38 per 100,000, we get, for the amount of solid matter annually carried mechanically to the sea by rivers, 1,043 cubic kilometres. That is the effect of the mechanical action of the continental waters.

What part do the waves of the ocean take in this action? When we hear the noise of the waves breaking against the bluffs and throwing their grape-shot of pebbles against them, and when we witness the enormous land-slides of which the sea-shores are often the theatre, we are sometimes led to think that the action of the sea is a preponderant factor in the destruction of continents. But the reverse is the case.

England may be regarded as one of the countries in which attacks by the sea upon the coast are most intense; for the waves of the Atlantic are thrown very violently against the shore by the southwest winds. English geologists appear to be agreed in thinking that the waste of the coasts of Britain under the action of the sea is certainly not more than three metres a century. It is true that at certain points of the French littoral, as at Havre, the banks are estimated to lose a quarter of a metre a year. M. Bouquet de la Grye raises the loss to a little more than a metre a year on the limestone coasts of the southwest; but, in compensation, there are seas where the work of the waves may be neglected as null, as well as flat coasts, where the sea, building up littoral bars, adds instead of taking away. I believe, then, till the contrary is proved, that if we allow for the whole earth a waste of three metres in a hundred years, we are above rather than below the truth.

If we suppose that the shore-banks average fifty metres in height, it follows that an annual waste of three centimetres will remove a cubic metre and a half per running metre, or 1,500 cubic metres per kilometre. The extent of the sea-coasts of the earth can be easily calculated with the aid of the figures given in Elisée Reclus's Continents, showing the proportion, in each continental unit, of dry-land surface and extent of coast-line. Applying these figures to those which represent the surface, now well