Open main menu

Page:Popular Science Monthly Volume 39.djvu/258

This page has been proofread, but needs to be validated.


I PURPOSE to inquire briefly into the probable future of the dry land, to ask if it is not destined to disappear, and to estimate the time that may be required to execute a sentence of extinction against it. It would have been hazardous to touch upon this question a few years ago. Precise data were wanting as to both the value of the relief of the land and the intensity of the actions which are called into play to change it. But the progress of geographical study has now put us in possession of more exact information, enough to permit us to seek a solution of the problem, not in the expectation of getting exact figures, but of calculating approximately the magnitude of the effects which we have to contemplate.

The labors of geographers in later years have given us a much more complete knowledge than we had before of the land relief. Ten years ago we still accepted Humboldt's estimate that, if all the asperities of the land were leveled over its entire surface, the resultant plateau would stand 305 metres above the surface of the sea. This figure began to grow perceptibly about 1880. A German student, Herr Krümmel, raised it to 444 metres. A few years ago, I thought it best, in preparing the chapter in my Traité de Géologie bearing upon this subject, to go into new calculations on the basis of existing hypsometric maps, and I came to the conclusion that the mean altitude of the dry land would be more than 500 metres, and would probably approach 600 metres. I declared this result with some reserve, on account of its novelty. But I had the satisfaction of seeing it immediately accepted by foreign geographers, and my estimates have since been exceeded; for Messrs. John Murray, Penck, Supan, and De Tillo, having been able, by the aid of the cartographic documents accessible to them, to make still more precise calculations, have found that the land relief may be represented by a uniform plateau rising to 700 metres above the level of the sea.

This plateau of 700 metres is the object of incessant attacks by the ocean on one side and atmospheric agents on the other. The rivers never cease carrying to the sea the fine fragments of the rocks which the rain washes into them, after they have been disintegrated by the alternate actions of moisture and drought, cold and heat, freezing and thawing. By observation of what takes place at the mouths of rivers, we may succeed in reaching a clear idea of the measure in which the silent action of atmospheric agents

  1. Address before the Geographical Society of Paris.