GEOPHYSICAL LABORATORY.^ 



Arthur L. Day, Director. 



Both science and story have left the interior of the earth virtually a closed 

 book. Imaginative writers have forecast many of the conquests of the sea and 

 of the air and of the regions beyond, but the interior of the earth remains inac- 

 cessible to them; scientific effort has often been consciously and even eagerly 

 directed toward it, but has revealed scarcely more. The distance from the 

 surface to the center of the earth is some 4,000 miles, of which hardly more 

 than a mile has been actually explored. What properties or what sub- 

 stances even a few more miles might reveal is a matter for inference alone, 

 inference for the most part with a very inadequate if not an insecure founda- 

 tion of fact. And, further, the number of productive inferences is so small 

 and their scope is so limited that they furnish little material or encouragement 

 for more elaborate study. 



We learned, for example, long ago from the astronomers that the density 

 of the whole earth as a body and a member of our solar system was about 

 five and one-half times that of water, while the geologists, studying its sur- 

 face, found only rocks of a density varying from two and one-half to three and 

 a quarter times that of water. The part of the earth which we know is 

 much lighter than the average of the whole, and the unknown interior 

 must therefore be different and heavier. The study of the tides and of the 

 figure of the earth, and even more particularly the discovery and measurement 

 of a tidal motion in the crust of the earth itself, revealed the fact that the 

 earth structure must be about as rigid as steel, from which it is a short step 

 to infer that the major portion of the interior may be of iron; we do not know. 

 Its density (about 8) would seem appropriate. Some further support has 

 been found for this inference from the nearly universal distribution of iron in 

 all known rocks and from the fact that meteors often come to us which are 

 composed almost entirely of it. The manner of transmission of earthquake 

 shocks through the earth may perhaps be considered to offer some support 

 also, though here the interpretation is more hazy. 



We also know that the temperature within the earth increases at a fairly 

 regular rate as we go down. In most regions it has already become too hot for 

 human activities at the depth of a mile and in some localities very much 

 sooner than that, so that no amount of boring skill will ever enable us to visit 

 the earth's interior. We must recognize as a further consequence that this 

 continual increase in the temperature downward and the parallel increase 

 in the weight of the overlying mass will speedily bring us to temperatures 

 and pressures which are also beyond the reach of experimental study in the 

 laboratory, and in consequence of this the effects of such pressures and 

 temperatures upon the earth-forming materials at great depths can not be 

 clearly predicted. 



Most fruitful in possibilities for future thought and exploitation, because 

 it directly concerns the 40 or 50 miles of the earth's shell next below the 

 very thin outer one which our mine-shafts reach, is the recently proposed 

 theory of the isostatic adjustment, within the crust of the earth, of differences 

 of loading, due (1) to the mountains and the gradual transfer of their material 



^ Situated in Washington, District of Columbia. 



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