xxvi Appendix. 



a depth of somewhere about thirty-live miles has a temperature below the 

 melting point of ordinary rocks at the surface, while ail below this depth has a 

 temperature above its melting point at the surface. Consequently, we have an 

 outer crust in which the attraction of cohesion among the molecules is greater 

 than the repulsion caused by heat, surrounding a nucleus in which the 

 repulsion caused by heat among the molecules is greater than the attraction of 

 cohesion. 



The outer crust must therefore be more or less rigid, while the superheated 

 interior must be in such a state that if the pressure that keeps it in its place 

 is decreased at one point it will expand, and this expansion will permeate 

 through the whole mass until the pressure is again equally distributed 

 throughout. Conversely, if the pressure is increased on any point, this 

 pressure will affect the whole mass and distribute itself evenly through it. Of 

 course I need hardly say that the rigid state of the crust is not separated from 

 the superheated state of the interior by a marked division, but the one passes 

 imperceptibly into the other. Now each poi-tion of this rigid crust must be 

 maintained in its place by three forces, viz. — its weight, the lateral thrust of 

 the arch, and the outward pressure of the superheated interior. While these 

 three forces remain constant equilibrium will be maintained, and no move- 

 ments will occur on the surface. But if one or more of these forces change in 

 amount, the equilibrium will be subverted and movements of the surface will 

 take place. If also the equilibrium be disturbed at one place, it follows, from 

 what I have said about the distribution of pressure in the superheated interior, 

 that the equilibrium will also be disturbed in all surrounding areas. If, for 

 instance, an upheaval of the crust should take place at any point, the under- 

 lying superheated rocks, being thus relieved from the pressure above them, 

 would expand and rise up, and till the hollow ; but this expansion would 

 spread through the mass, and would therefore lessen the outward pressure of 

 the interior in all tlie surrounding areas, which would consequently subside, 

 and equilibrium would only be once more restored when the mass of the 

 subsided ai*eas equalled the mass of the elevated area. Consequently elevation 

 implies subsidence, and vice versd. Where now must we look for the causes 

 that are in operation to disturb this equilibrium ? The most obvious is the 

 radiation of heat into space by the earth, and the consequent cooling and 

 contraction of the superheated interior. This is at present almost universally 

 accepted by geologists as the cause of the movements of the sui-face and the 

 upheaval of mountain chains, but many arguments have been urged against it, 

 and although I am willing to allow that it must have some effect in producing 

 movements, these effects are, I think, completely absorbed by the much 

 larger ones that flow from causes that I shall presently describe ; iind it is 

 quite impossible that it can be the only cause of movement, partly because 



