i873-] Condition of the Moon's Surface. 45 



forces called into play by the contraction of our globe. Ac- 

 cording to the new theory, it is not the pressure of matter 

 under the crust outwards, but the pressure of the earth's 

 crust inwards, which produces volcanic energy. Nor is this 

 merely substituting an action for reaction, or vice versa. 

 According to former views, it was the inability of the crust 

 to resist pressure from within which led to volcanic explo- 

 sions, or which produced earthquake throes where the 

 safety-valve provided by volcanoes was not supplied. The 

 new theory teaches, in fact, that it is a deficiency of internal 

 resistance, and not an excess, which causes these dis- 

 turbances of the crust. " The contraction of our globe," 

 says Mallet,* " has been met, from the period of its fluidity 

 to its present state, — first, by deformation of the spheroid, 

 forming generally the ocean-basins and the land ; after- 

 wards by the foldings over and elevations of the thickened 

 crust into mountain-ranges, &c. ; and, lastly, by the me- 

 chanism which gives rise to volcanic actions. The theory 

 of mountain elevation proposed by C. Prevost was the only 

 true one, — that which ascribes this to tangential pressures 

 propagated through a solid crust of sufficient thickness to 

 transmit them, these pressures being produced by the 

 relative rate of contraction of the nucleus and of the crust ; 

 the former being at a higher temperature, and having a 

 higher coefficient of contraction for equal loss of heat, tends 

 to shrink away from beneath the crust, leaving the latter 

 partially unsupported. This, which during a much more 

 rapid rate of cooling from higher temperature of the whole 

 globe, and from a thinner crust, gave rise in former epochs 

 to mountain-elevation, in the present state of things gives 

 rise to volcanic heat." By the application of a theorem of 

 Lagrange, Mr. Mallet proves that the earth's solid crust, 

 however great may be its thickness, " and even if of mate- 

 rials far more cohesive and rigid than those of which we 

 must suppose it to consist, must, if even to a very small 

 extent left unsupported by the shrinking away of the nucleus, 

 crush up in places by its own gravity, and by the attraction 

 of the nucleus. This is actually going on ; and in this 

 partial crushing," at places or depths dependent on the ma- 

 terial and on conditions which Mr. Mallet points out, he 

 discerns "the true cause of volcanic heat.t As the solid 



* I quote throughout from an abstract of Mallet's paper in the " Philoso- 

 phical Magazine" for December, 1872. The words are probably, for the most 

 part, Mallet's own ; but I have not the original paper by me for reference. 

 I believe, however, that the abstract is from his own pen. 



f, " In order to test the validity of his theory by contact with known fads" 

 (says the "Philosophical Magazine"), "Mr. Mallet gives in detail two im~ 



