Page:Popular Science Monthly Volume 15.djvu/331

 being required to remove a foot of surface—that solidification by cooling must keep pace with it; 2. Volcanoes are generally not situated in areas of erosion, but along coast-lines and on islands; and, 3. They are conspicuously associated with lines of fracture and elevation.

A simple explanation of the phenomena of vulcanism is suggested by the writer, and that is—the relief of pressure by slight arching of the crust of the earth along lines of elevation, while the pressure is maximum under the unbroken areas on either side. This unequal pressure would cause a flow of liquid or viscous matter toward and upward under the mountains that mark the lines of arch and fracture, and would permit heated matter held in solidity by pressure to assume the fluid state.

Mallett's theory, that the arching of plates of the earth's crust, and the arrest of their motion in falling, would generate heat sufficient to liquefy masses of rock, and produce volcanic eruptions, is rejected, for the reasons that the strain on arches of sufficient magnitude would be too great for the resistance of the materials composing them, and as Mr. Fisher argues, the heat generated by this method, even if great enough, would not be localized.

Mallett's theory wag framed to account for vulcanism in the crust of the earth, on the supposition that the crust was very thick, as claimed by Hopkins, Thompson, and Darwin, but Henessey and Delaunay have clearly shown that the investigations supposed to demonstrate the great thickness of the crust are valueless and irrelevant, as the premises assumed are not those of nature, and that we have as yet no evidence of such thickness of crust as would make it impossible for volcanoes to be fed from a general molten mass below the earth's solid crust.

In conclusion, we take pleasure in commending the numerous maps and plates which embellish these elegant volumes. No other scientific work known to us has so many, nor any more artistic or better adapted to supplement and illustrate the text.



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