ADVISORY COMMITTEE ON GEOPHYSICS 45 



2. Some of the problems of geophysics. — To illustrate this view of 

 the matter brief comments may be submitted on three of the many- 

 great questions of geophysics, namely, the distribution of terrestrial 

 density, upheaval and subsidence, and vulcanism. All of them are 

 subjects of extreme difficulty, but not, in my opinion, beyond the 

 reach of well directed efforts. 



The distribution of densities evidently depends upon the mate- 

 rials of which the globe is composed, the temperatures which pre- 

 vail at different depths, and the elastic properties of matter The 

 possible laws of terrestrial density cannot be intelligently discussed 

 beyond the point where L,aplace left the subject until the law of 

 elasticity for finite stresses has been elucidated. 



Some suggestive work on elasticity has recently been done in the 

 Division of Chemical and Physical Research of the U. S. Geologi- 

 cal Survey along the lines of a former investigation of my own, and 

 is stiil in progress there. While a large amount of confirmatory 

 experimentation is still requisite, enough has been accomplished to 

 indicate the character of the result. 



The law of elasticity for constant temperatures having been dis- 

 covered, it will then be necessary to determine the functional rela- 

 tions of elasticity and temperature. And since the range of tem- 

 perature is necessarily large, considerable researches at high 

 temperatures must accompany the study of the finite stress-strain 

 function . 



Supposing the relations known, a comparison of the earth, the 

 moon and Mars can be employed to test the more probable theories 

 of composition ; for example, that which considers the planets, like 

 the meteorites, to be composed of nickel -steel and stony matter com- 

 parable with the average rock . 



The theory of the vibrations of a sphere, when adequatelj^ de- 

 veloped, can be used still further to confirm or to upset working- 

 hypotheses as to the chief terrestrial constituents by taking advan- 

 tage of the fate of transmission of earthquake shocks along 

 various chords of the terrestrial sphere. 



Evidently therefore the problem of the distribution of densities 

 requires the combined efforts of at least three investigators — one de- 

 voted to elasticity, a second to high temperatures, and a third to 

 the mathematical development. 



The most hopeful line of attack on the problem of upheaval and 

 subsidence appears to me to be as follows : A sphere is conceivable 

 in which the distribution of thermal diflfusivities and conductivi- 



