360 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 31 



outer earth shells. Time fails for us to enter on a discussion of what 

 is known as seismic prospecting, by means of which commercial 

 interests probe the upper 4,000 feet or so of the earth's crust in the 

 search for oils and minerals. They delineate many of the details of 

 the upper layers, but, from a purely scientific standpoint, the most 

 interesting result is the large mass of data showing the relation of 

 the wave velocity and the type of material traversed. 



AVe have outlined, then, the present theory of the structure of the 

 interior of the earth and indicated the means by which it is to be 

 checked and improved through a study of seismic waves. If a later 

 modification of the theory should give traveltime curves more nearly 

 in accord with later and more accurate data, the modification will be 

 adopted. The present theory is (in the language of the automobile 

 prospectus) the latest model, which we take pleasure in exhibiting at 

 this time. We hope it will find its way into the hands of many and 

 that they may enjoy the fullest service in its use. It may be traded 

 in as soon as suggested improvements have been found worthy of 

 adoption. All may rest assured that when better theories of the 

 internal structure of the earth are built, seismology will build them. 



