SHAPING THE EAETH — BOWIE 339 



minerals and their effect on geological processes, are inclined to the 

 oi:)inion that the deep-lying materials have practically no effect on 

 surface changes. 



If we eliminate forces existing outside of the earth, forces due to 

 the supposed contraction of the earth's nucleus and the collapsing of 

 the crust, and forces due to the effect of radioactive minerals as 

 major causes of earth movements, we must search for some other 

 forces that might be effective. 



We know that the temperature of the earth increases with depth. 

 For the first 2 miles or less we have definite data from the deter- 

 minations of temperatures in wells. There is a great variation in the 

 rate at which the temperature increases with depth, but a fair aver- 

 age is 50° C. per mile. The temperature certainly continues to 

 increase below the 2-mile depth, for we have many active volcanoes 

 in the world which emit cinders and lavas having temperatures of 

 1,000° C. or more. Such temperatures would be found at a depth 

 of approximately 20 miles if the temperature gradient were about 

 the same throughout that depth as it is near the surface. Whether 

 the temperature keeps on increasing with depth down to the center 

 of the earth, we can not tell, for there is no way to discover, even 

 approximate!}^, what the temperature may be at great depths. A 

 material may be at a temperature which at the surface would be its 

 melting point or even its boiling point, yet it probably would act 

 like a strong solid when confined by the great pressures which must 

 exist at considerable depths. A very hot interior of the earth, if 

 there is little change in temperature from one period of time to 

 another, will not exert any decided influence on the configuration 

 of the earth's surface. Change in heat, however, whether a de- 

 crease or increase, will exert force. It will cause expansion or con- 

 traction of materials, but the heat of the interior of the earth is 

 changing so slowly that it can not be a major cause of surface 

 changes. One would be most unwise to assert that the heat of the 

 interior of the earth, without any other influences acting, could not 

 cause changes in elevation and geographic positions of points on 

 the earth's surface, but, if this interior heat is a primary cause of 

 surface movements, no one, so far as I am aware, has given a very 

 clear explanation as to how the changes are effected. I am rather 

 inclined to think that we may eliminate the heat of the earth's 

 interior as the major cause of geological phenomena. This heat does 

 affect those portions of the crust which are lowered by sedimen- 

 tation or raised by erosion, but it is not the primary cause of surface 

 movements. I believe we should look for something that is closer 

 at hand and easier of understanding. 

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