Royal Society. 469 



mechanism which he points out as giving rise to volcanic action. 

 The theory of mountain-elevation proposed by C. Prevost was the 

 only true one, — that which ascribes this to tangential pressures pro- 

 pagated through a solid crust ot sufficient thickness to transmit them, 

 those pressures being produced by the relative rate of contraction of 

 the nucleus and of the crust : the former being at the 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, the author proves that 

 the earth's solid crust, however great may be its thickness, and even 

 if of materials 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 material and on conditions pointed out, 

 the author discovers the true cause of volcanic heat. As the solid 

 crust sinks together to follow down after the shrinking nucleus, the 

 work expended in mutual crushing and dislocation of its parts is 

 transformed into heat, by which, at the places where the crushing 

 sufficiently takes place, the material of the rock so crushed and of 

 that adjacent to it are heated even to fusion. The access of water 

 to such points determines volcanic eruption. Volcanic heat, therefore, 

 is one result of the secular cooling of a terraqueous globe subject to 

 gravitation, and needs no strange or gratuitous hypothesis as to 

 its origin. 



In order to test the validity of this view by contact with known 

 facts, the author gives in detail two important series of experiments 

 completed by him : — the one on the actual amount of heat capable 

 of being developed by the crushing of sixteen different species of 

 rocks, chosen so as to be representative of the whole series of 

 known rock formations from Oolites down to the hardest crystalline 

 rocks ; the other, on the coefficients of total contraction between 

 fusion and soHdification, at existing mean temperature of the atmo- 

 sphere, of basic and acid slags analogous to melted rocks. 



The latter experiments were conducted on a very large scale ; and 

 the author points out the great errors of preceding experimenters, 

 Bischoff and others, as to these coefficients. 



By the aid of these experimental data, he is enabled to test the 

 theory produced when compared with such facts as we possess as 

 to the rate of present cooling of our globe, and the total annual 

 amount of volcanic action taking place upon its surface and within its 

 crust. 



He shows, by estimates which allow an ample margin to the best 

 data we possess as to the total annual vulcanicity of all sorts of 

 our globe at present, that less than one fourth of the total heat at 



