IMPRISONED HEAT. 101 



have not been fused and reduced to lava, but have been soft- 

 ened and vitrified, and afterward cooled. Then the daily radi- 

 ation of heat from the earth exceeds the amount received from 

 the sun. If the earth is cooling, and has for geologic periods 

 been cooling, it is not difficult to admit that some former tem- 

 perature was high enough to reduce it to a molten condition. 

 If that condition existed, the process of ccoling would result 

 in a film over the exterior, which would be the primitive or 

 fire-formed crust, on which the first ocean descended, and the 

 first sediments accumulated, while the protected interior re- 

 tained a higher temperature. The fusing temperature now 

 existing within may be but the residuum of primitive heat left 

 after so long a process of cooling. This is one view. 



Again, it has been contended that the internal heat results 

 (at least in part) from crushing and friction in the crust, pro- 

 duced by motions and pressures exerted. Mallet showed that 

 the heat generated in crushing small cubes of granite might 

 become sufficient to cause fusion. We know, also, that the 

 cohesion of no substance is adequate to withstand all assigna- 

 ble pressures. No rock has the requisite rigidity to resist the 

 crushing weight of a mountain twenty miles high. Whatever 

 movements may take place in the earth's crust, involve masses 

 so great and forces so enormous that the resistances of ordinary 

 matter are inconsiderable. The most solid rocks are essentially 

 fluid or viscid. Now, such moyements must necessarily result 

 from two causes : First, a slow shrinkage of the earth through ; 

 loss of heat ; secondly, the attractions of the sun and moon, 

 which cause tidal protuberances on the surface of the earth, 

 however rigid it may be ; and these, continually shifting their j 

 positions, as the oceanic tides do, result in daily motions ade- 

 quate to develop a large amount of frictional heat. 



But, whatever the cause of the internal heat existing, we 

 can not demonstrate that the whole interior is molten ; nor that 

 the earth is solid to the core ; nor that we have a solid core and 

 a solid crust, with an irregular zone between, in which the 

 matter is molten, or, at least, in a plastic state. We have 

 many facts; we are building our theories cautiously, and in the 



