22 TRAKSACTIONS OF THE [OCT. 18,. 



effect produced by the attraction of the sun and moon — have 

 led to the conclusion that the earth's crust is thicker than this, 

 and it has been suggested that while the accuracy of the obser- 

 vations on the increase of temperature to the depth of one mile 

 cannot be questioned, it is possible that the rate of increase to- 

 that depth is not constant below; that the increment may 

 diminish, and therefore the temperature of fusion may only be 

 reached at a greater depth than has been supposed. 



It has also been shown that the melting point of many sub- 

 stances is raised by pressure, and therefore, that the enormous 

 weight of the overlying rocks, equivalent to 792,000 lbs. to the 

 square foot for every mile, may hold in coerced rigidity a con- 

 siderable zone of the earth's mass composed of materials that 

 would melt and flow on the surface at a much lower temperature 

 than that which they now endure in a solid form. Accepting, 

 then, the conditions imposed on the old theory of the state of 

 the interior of the earth by pressure and the possible diminution 

 of the increment of temperature, we may suppose that the solid 

 crust is considerably thicker than was formerly supposed. That 

 it is relatively thin, however, is indicated by facts which will be 

 cited further on. 



The increase in temperature observed in mines and deep bor- 

 ings means that the heat of the interior of the earth is constantly 

 escaping to the surface, where it is radiated into space. If the 

 outer crusts were a perfect non-conductor, the materials within 

 it would always maintain a condition of thermal equilibrium 

 throughout. It is thus evident that the process of refrigeration 

 is progressive, and from the time when the first film of solid 

 matter dimmed the brightness of the "glittering globe of liquid 

 fire," the crust formed at the surface has been constantly 

 increasing in thickness, while by the loss of heat, which is an 

 expansive force antagonistic to gravity, the volume of the earth 

 has been as constantly diminishing. But since the outer crust 

 has lost its inherent heat and has become solid it no longer 

 shrinks, though the loss of volume goes on incessantly in the in- 

 tensely heated, but gradually cooling interior. As the nucleus 

 contracts the solid crust cannot accommodate itself moment by 

 moment to the loss of volume, for it resists by its rigidity and is 

 brought into a state of strain. This is relieved from time to 

 time, whenever it passes the resistance of the materials compos- 

 ing the crust, by a crushing together and displacement of the 

 surface rocks. These a,vQ faulted or folded ; that is, are either 

 thrown into great waves by lateral pressure, or the arches are 

 broken and fissures are produced at right angles to the line of 

 thrust. The rocks forming the sides of these fissures slide on 

 each other, forming what geologists call faults, in whicli the 



