SHAPING THE EARTH BOWIE 333 



may be deeper under the continents than under the oceans. The 

 compensatino^ deficiency of density under a mountain system may 

 be confined to a rather narrow zone vertically and not extend 

 throughout the thickness of the crust. The compensation may be 

 distributed widely in a horizontal direction from the topographic 

 feature, and deficient densities under land masses and excessive densi- 

 ties under ocean areas may not be sufficient to balance the irregulari- 

 ties of the earth's surface in the regions studied. Finally, the den- 

 sity of surface rock is variable. Undoubtedly, all of these factors 

 come in to cause the differences between the theoretical and actual 

 values which we call anomalies, but the anomalies are so small after 

 the isostatic method has been applied that investigators are inclined 

 to believe that the principle of isostasy has been amply tested and 

 proved. Some of them, and I am one, believe that the principal 

 cause of the anomalies is the effect of abnormally heavy or light 

 material near the earth's surface and close to the astronomical or 

 gravity stations. If we could find out the actual distribution of 

 density in the earth's materials for a depth of 5 or 10 miles below 

 the earth's surface, I am confident that we could reduce nearly all 

 of the anomalies. 



This brings up the question as to whether or not it would be pos- 

 sible to discover what the geologists call structural features that are 

 buried below the earth's surface. This is a matter of great impor- 

 tance and may have a bearing on the search for petroleum and ores. 

 The gravity survey conducted over this country indicates certain 

 places where there are extra heav}'' or extra light masses of material 

 fairly close to the earth's surface. I do not know of any oil having 

 been found, or drilling for oil having been undertaken, near any of 

 our gravity stations as a result of our data, but I am sure that an 

 intensive gravity survey would disclose structure that might be of 

 value in the oil and mining industries. 



SOME ISOSTATIC CONCLUSIONS 



The evidence seems to justify the conclusion that all mountain and 

 plateau areas were at one time occupied by low lying portions of the 

 earth's surface on which great beds of sediments were laid down. 

 Then these areas were raised up to form either mountains or plateaus. 

 If there was much distortion, mountains resulted, and if the area 

 went up in a more or less uniform way, extensive plateaus were 

 formed. What caused these uplifts is one of the outstanding prob- 

 lems of the science of geology. Many of the investigators of the past 

 have postulated horizontal thrusts, while some, Dutton included, 

 were inclined to favor a vertical movement as the predominant one 

 with horizontal movements as incidental. 



