26 president's address. 



liberal allotment of time, and there is no reason for limiting the 

 original supply of radium-producing material, and, hence, of the 

 time during which its heat has been available. 



It is quite obvious that in the earlier stages of the earth's 

 history, when rapid cooling was taking place, crust movements 

 on a colossal scale must have occurred, and that as cooling pro- 

 ceeded, these would gradually moderate. If we imagine a time 

 when all this heat had disappeared and the earth had arrived at 

 a stage of thermal equilibrium such as is assumed under the 

 extreme radium theory to be its existing condition, it is evident 

 that no more shrinkage could take place and that any display of 

 crust movement or volcanic energy must be due to some other 

 cause. The material of the earth's interior up to a point com- 

 paratively near the surface, is, as we know, at a temperature con- 

 siderably above its surface melting point, but there is evidence 

 that it is retained in a solid state by the pressure of the super- 

 natant strata. Whenever this pressure is relieved liquefaction 

 occurs, and we then have the fused matter squeezed out in the 

 form of lava, through any available opening; or, it may be, form- 

 ing sheets or dykes at or beneath the surface. We may consider 

 any given land area as floating on a substratum of lava, which, 

 though solid, or perhaps more or less plastic, is ready to respond 

 to relaxation of pressure. It is thoroughly well understood that 

 the surface of the land, and, in particular, the great mountain 

 masses, lose in the aggregate very large quantities of material 

 every year through denudation. The removal of such quantities 

 of matter from one place to another on the earth's surface must 

 have a very considerable effect on regional stability, and w411 be 

 quite competent to account for extensive earthquake and other 

 movements. When a land surface is stripped by denudation and 

 the material so removed deposited around it, w4iile the pressure 

 of the surface in question is lessened, that of the area receiving 

 the spoil is increased, and the effective force operating in the 

 direction of raising the one area and depressing the other will, 

 in an ideal case, be double the weight of the transferred material. 



