Page:Popular Science Monthly Volume 39.djvu/261

Rh a reassuring figure to us. But the geologist, who looks at the past as well as at the future, far beyond existing generations, can draw more than one lesson from it. First, the whole history of the globe not being included in a space of time relatively so short, the result teaches us that its equilibrium has more than once been troubled by great phenomena of dislocation, too rare, however, for it to be possible for man to have been a witness of them, which, building up new reliefs as barriers to destruction, have given new impulses to the action of natural forces.

On the other hand, geological observations furnish a tolerably approximate measure of the maximum thickness of the deposits that are made in the bottom of the sea. The total thickness amounts, according to Dana, to 45,000 metres. To learn how long a time may have been occupied with the formation of such deposits, let us seek to represent to ourselves what now becomes of the products of the destruction of the continents.

These deposits, it is now known, do not extend, by a great deal, over the whole surface of the sea bottom; but they form a zone of strata which the deep-sea sounding expeditions have enabled us to define fairly well. According to Mr. John Murray's estimate, the sediments formed by the destruction of the continents spread themselves over about a fifth of the oceanic surface. Thus, although the oceanic area is superior to the surface of the land, the mass of the deposits distributing themselves over only a fraction of the extent, there may result, at the end of 5,000,000 years, an accumulation of sediments capable of forming a body 750 metres thick. But this thickness would certainly be very unevenly distributed; almost null at the finishing point of the deposits in breadth, its thickness would be much greater near the coasts, and it would not be hazardous to suppose that it might rise there to 2,000 or 3,000 metres. To realize the total thickness of 45,000 metres—that is, to explain geological history—it would be sufficient to suppose that the life of the globe has included some fifteen or twenty periods of 4,500,000 years, or from 67,000,000 to 90,000,000 years, a number a little less than the 100,000,000 years which Sir William Thomson has calculated upon estimates of the loss of internal heat.

The objection may be brought up that I have neglected in this calculation the contributions of volcanic action to the land relief, which it is thought should be counted in attenuation of the destructive effect of running waters. We owe to Cordier a calculation that the lavas which have been thrown up during the historical period represent at most 500 cubic kilometres, or, counting that period at 3,000 years, a sixth of a cubic kilometre per year. This is a very little affair compared with the amount of the waste which I have pointed out. We should likewise recollect that