310 The Rev. 0. Pisher on Mr. Mallet's 



towards the centre of figure. The interior goes on contracting 

 until the compression accumulates sufficiently to cause a move- 

 ment of some kind among the particles of the crust, be it crush- 

 ing, or faulting, or corrugation, or what not; and the motion 

 being arrested, the work becomes transformed into heat. This 

 heat, as I have already proved, can be developed only at those 

 places where the movement occurs. The question is whether it 

 will be sufficient to give rise to a volcano. If some places are 

 weaker than others, the movement will necessarily occur at them. 

 Let there be several such, which for simplicity we will suppose 

 to be situated at points upon a great circle. The force of com- 

 pression will go on increasing until one of these places gives 

 way. Let P x be a general symbol for this increasing force, 

 measured by the pressure, in terms of the weight of a cubic mile 

 of mean rock at the surface, upon a square mile of vertical sec- 

 tion of the crust. 



Now it is evident that no greater lateral shortening, or ap- 

 proach of the particles of the crust among themselves, can take 

 place at one place than at another, except through a lateral 

 movement of the layers of the crust over the nucleus towards 

 the place in question. Hence any' localization of work at a 

 given place must require this to occur more or less. Conse- 

 quently if the pressure goes on accumulating until it becomes 

 equal to P x all round, and under that pressure the crust begins 

 to yield at A, the yielding there cannot relieve the pressure any- 

 where else, unless the crust is shifted over the nucleus towards A. 



Having premised thus much, we will endeavour to find the 

 amount of heat which can be developed along a vertical section 

 at any one place in the crust under several conditions. 



Conceive a strip of the crust, one mile in width, situated 

 along a great circle ; and suppose the pressure to have gone on 

 increasing to some value P x , which has caused the crust to yield 



BAB' 



at A; and that by such yielding the pressure has been reduced 

 to P A at A, and relieved over AB and AB' on either side of A, 

 but not beyond B and B'. Hence at B, B' the pressure will 

 still be P x . Beyond B and B' it will have been relieved by 

 yielding at other places ; and it is supposed that it does so yield 

 all round, so as to fit the reduced nucleus. Hence the amounts 

 to which the pressure will accumulate at different places will de- 

 pend upon the strength of the crnst ; but the amount of shorten- 

 ing of the whole, crust will not depend upon the amount of the 

 pressure, but solely upon the amount of contraction of the nu- 

 cleus. Consequently if e be the coefficient of linear or radial 



