i6 The American Geologist, January, 1905 
help and in most cases meets with the reply : We can not 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 astro- 
physics of this, the only accessible planet. Geodesy, too, and 
terrestrial magnetism are waiting for the solution of geo- 
physical 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 in- 
dividual 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 completely solved, though in many cases we have 
some inkling of the solution. 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 properties 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 log- 
ical 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 applicable at least to the moon and Mars. 
No one questions that enormous pressures and very high 
temperatures exist near the earth's center, while the quality 
of matter which constitutes the interior can not be satisfac- 
torily determined until we know how substances would be- 
have under extreme pressure and at temperatures approach- 
ing 2,000 degrees C. There is every reason to suppose that 
