366
period of its fluidity to its present state,—first, bydeformation of the spheroid, forming generally theocean-basins and the land; afterwards by the foldingsover and elevations of the thickened crust into moun-tain-ranges, &c.; and, lastly, by the mechanism whichgives rise to volcanic actions. The theory of moun-tain-elevation proposed by C. Provost was the onlytrue one,—that which ascribes this to tangentialpressures propagated through a solid crust of sufficientthickness to transmit them, these pressures beingproduced by the relative rate of contraction of thenucleus and of the crust; the former being at a highertemperature, and having a higher coefficient of con-traction for equal loss of heat, tends to shrink awayfrom beneath the crust, leaving the latter partiallyunsupported. This, which during a much more rapidrate of cooling from higher temperature of the wholeglobe, and from a thinner crust, gave rise in formerepochs to mountain-elevation, in the present stateof things gives rise to volcanic heat.” By the ap-plication of a theorem of Lagrange, Mr. Mallet provesthat the earth’s solid crust, however great may be itsthickness, “and even if of materials far more co-hesive and rigid than those of which we must supposeit to consist, must, if even to a very small extent leftunsupported by the shrinking away of the nucleus,crush up in places by its own gravity, and by theattraction of the nucleus. This is actually going on;and in this partial crushing,” at places or depthsdependent on the material and on conditions which