Notices of Memoirs— Hutton, on Mountains. 167 



acquainted, and it is one wliich. has never yet received a satisfactory 

 explanation. I hope, however, to be able to show you that it is but 

 the necessary effect of causes which we know from observation to be 

 constantly going on on the surface, combined with the conduction 

 outwards of the interior heat of the earth. 



In order to make what I have to say quite clear to you, I must 

 first briefly refer to some general considerations on the interior of the 

 earth. Fortunately, it will not be necessary for me to enter into the 

 hotly disputed question as to whether it is fluid or solid, for this is 

 immaterial to the views that I have to advance ; all that is necessary 

 being that the interior is very hot. This is allowed, I believe, by 

 all scientific men, the proof resting principally on the facts that we 

 know from observations, wherever they have been made, that the 

 temperature actually does rise as we descend, at an average rate of 

 about 1° Fahr. for every fifty feet, and that the density of the earth 

 is so small, not much more than twice that of the ordinary rocks of 

 the surface, that there must be some expansive force in the interior 

 sufficiently powerful to balance in a great measure the enormous 

 pressure to which the interior of the earth would be subjected. 

 Assuming then that the interior of the earth is intensely heated, and 

 that the temperature, for a depth say of fifty miles from the surface, 

 increases at the rate of 1° Fahr. for each fifty feet, it necessarily 

 follows that the outer shell, or " crust," as it is commonly called, to 

 a depth of somewhere about thirty-five miles, has a temperature below 

 the melting point of ordinary rocks at the surface, while all 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 super- 

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

 imperceptibly into the other. Now each portion 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 super- 

 heated interior. While these three forces remain constant, equilibrium 

 will be maintained, and no movements 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 



