PRESENT PROBLEMS OF GEOPHYSICS 517 



succeeded in ascertaining the quantitative relations between effects 

 and operative causes, and have been perforce content to indicate 

 tendencies. Thus geological doctrine is far too much a matter of opin- 

 ion, but this is hardly the fault of the areal geologist. The country 

 must be mapped both for economic reasons and to accumulate a know- 

 ledge of the facts to be explained. Working hypotheses the field geo- 

 logist must have, or he could not prepare his map; and he is only 

 responsible for living up to the standard of knowledge of his time. 

 He is continually face to face with phenomena for which physics 

 and chemistry should account, though they have not yet done so, and 

 must accept seeming probabilities where certainty is unattainable. 

 So, too, Kepler's predecessors recorded facts and guessed at general- 

 izations as best they might. 



The physics of extreme conditions still awaits satisfactory explora- 

 tion. The geologist turns to the physicist for help, and in most cases 

 meets with the reply: We cannot tell. Astrophysics is in much the 

 same situation. Astronomers know as little of the distribution of 

 density in the stars or planets as do geologists. Real knowledge of the 

 physics and chemistry of high temperatures would be as welcome to 

 them as to us. After all, physical geology is the astrophysics of this, 

 the only accessible planet. Geodesy, too, and terrestrial magnetism 

 are waiting for the solution of geophysical problems. How much might 

 be done, Lord Kelvin and Mr. George H. Darwin have shown; but 

 there are many problems too broad and too laborious to be solved by 

 individual effort, and these are as essential to the rounding-out of the 

 science of physics as they are to the development of geology and 

 astrophysics. 



In the brief review which precedes, I have endeavored to show that 

 the history of the earth bristles with problems, few of them com- 

 pletely solved, though in many cases we have some inkling of the solu- 

 tion. This sketch has been drawn for the purpose of considering the 

 strategy of a campaign against the series of well-intrenched positions 

 occupied by our great enemy, the unknown. 



Generalizing the results of the sketch presented, it is easy to see 

 that nearly all the problems suggested involve investigation of the pro- 

 perties of solids, or of liquids, or of the transition from one phase to 

 the other. It is the business of the experimental physicist to establish 

 linear relations; it is the occupation of the mathematical physicist to 

 draw logical inferences from these relations. Each will have plenty 

 to do in a methodical study of geophysics. 



There can be no doubt that the character of the earth's interior 

 and the physical laws which there prevail constitute the most 

 fundamental object of geological and geophysical research, while 

 the results of successful investigation would be immediately appli- 

 cable at least to the moon and Mars. No one questions that enormous 



