206 O. C. JONES, GEORGE D. HUBBARD Vol. XXII, No. 7 



for steam, weakens the rocks, and the expansion caused by 

 increased temperature, amounting to 1 cm. to every 19.2 meters 

 for every 100 degrees C. rise of temperature, causes a sHght 

 disturbance which is carried on by the pressure from the sides; 

 erosion and deepening of the geosynchne tend to maintain 

 isostatic adjustment throughout the process. Now inertia may 

 carry the process beyond isostatic adjustment; certainly the 

 rock thus thrown up cools to some extent. Both things may 

 occur; at least there is always a period of normal faulting^- 

 succeeding a period of folding which might be caused by these 

 factors. The mountain making process may be aided by lateral 

 pressure developed by crustal expansion due to rising temper- 

 ature of the crust. This, however, is mere supposition, and 

 perhaps unsupported by geologic evidence. 



Faulting is another process through which equilibrium is 

 maintained. It differs from mountain making largely in that 

 rock flowage and bending are predominant there; whereas 

 faulting is the result of adjustment with fracture of the rocks 

 rather than flow. It is not usually associated with the deeper 

 synclines, although it is confined probably to regions of rather 

 deep sedimentations. 



Causes of lack of adjustment may be external, such as the 

 tidal influence of the sun or moon, the shifting of the rotational 

 axis, pressure of the atmosphere, increased load due to forma- 

 tion of glaciers, or the accompanying emptying of the seas, 

 great evaporation or excessive precipitation; but probably the 

 great cause of lack of adjustment is the shift of load due to 

 erosion. 



Earthquakes, so far as is known, present no opposition to our 

 thesis that all the dynamics of the crust are the result of pro- 

 cesses of adjustment of heterogeneous density to the form of the 

 earth through the forces of gravity. The fact that calculations 

 of depth of focus invariably indicate not a point but a plane 

 lying more or less vertical to the surface, and that many earth- 

 quakes actually accompany slipping along a fault plane, would 

 indicate that the phenomena of tectonic earthquakes are 

 invariably of this character. The fact that many earthquakes 

 show merely lateral stresses without elevation or depression in 

 no way militates against our general thesis. Lateral stresses 



'2T. M. Reade: Evolution of Earth Structure, p. 137-8. 



