Page:Popular Science Monthly Volume 41.djvu/418

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the compensating forces, he recognizes their existence and their title to be considered. If we understand him aright, his article was designed to be tentative, and as opening the way to a discussion in which much may be said on both sides.

The discussion which follows will help to give more light concerning M. de L'Apparent's views and meaning and their value.

M. Jacques Léotard has written in the Revue Scientifique that M. de L'Apparent has in his evidently very curious study arrived at his result only by neglecting several factors of contrary effect.

While it is admitted that the earth is swept by powerful atmospheric agents which, if their work was continued without compensatory action on the other side, would ultimately level and submerge all the continents, M. Ldotard insists that there exist other very important causes of increase of the relief, the action of which now counterbalances and may ultimately surpass that of the solvent influences.

One of these causes, of which M. de L' Apparent took some notice, is the contribution of volcanic products to the soil It is one of the most minute of the factors, but M. de L'Apparent's estimate of one sixth of a cubic kilometre a year seems too small. The three hundred known active volcanoes on the surface of the earth ought to give out a much larger quantity of their internal products; and it should be remarked that the dejections of craters, besides lavas, comprise various rocks, mud, and ashes. But the importance of this factor, little at the most, is made still less by the occurrence of volcanic explosions on the sea-coast, in which considerable tracts of land have been swallowed up.

The chief essential cause of increase of dry land at the expense of the ocean lies in the evolution of our planet. During the geological epochs of thousands of centuries each the upheavals which formed existing continents have come in gradual succession, taking from the primitive sea, which originally extended over the whole earth, a larger and larger part of its immense domain. These upheavals, under the action of internal forces, have continued slowly till our own time in many regions, notably in the north and center of the Scandinavian Peninsula, Spitsbergen, northern Siberia, Turkistan, Scotland, Sardinia, Tunis, on the coasts of the Red Sea, etc., while the depressions of vast countries, which must not be confounded with little local collapses produced by the subterranean work of water, are less numerous. Besides the increase of continents, new islands of volcanic origin rise at times to the surface of the seas, and lands are also gradually formed by the accumulation of sedimentary matter and organic remains. The deltas which rise at the mouths of large rivers in consequence of the deposition of mud and sand transported by the streams, likewise constitute an augmentation