Present Problems of Geophysics. — Becker. 7 
It is a very striking fact that the mean rigidity of the 
earth is about that of steel, for the only substances likely to 
occur in extensive continuous masses and displaying such 
rigidity at ordinary temperatures and pressures is steel itself. 
Nevertheless, the conclusion can not yet be drawn from the 
resistance to deformation displayed by the earth, that it is 
chiefly composed of steel. Elastic resistance is known to be 
a function both of pressure and of temperature, and until this 
function has been determined by theory and experiment, the 
bearing of the evaluation of rigidity by tidal action can not 
be ascertained. 
Having shown the earth to be a solid globe, lord Kelvin 
calculated its age from one of Fourier's theorems, assuming 
for purposes of computation an initial temperature of 7000 
degrees F. (nearly 3900 degrees C.) and that the thermal 
dififusivity of the earth is that of average rock. These assump- 
tions, with the observation that the temperature, near the 
surface of the earth increases at the rate of i degree F. 
for every 50 feet of depth, lead to an age of 98,000,000 years ; 
but on account of the uncertainty as to conductivities and 
specific heats in the interior, the conclusion drawn by lord 
Kelvin was only that the time elapsed since the inception 
of cooUng is between 20 and 400 million years. 
Clarence King subsequently took a further important 
step on the basis of data determined at his request by pro- 
fessor Carl Barus on the volume changes which take place 
in diabase during congelation, and on the eflfects of pressure 
in modifying the melting and solidifying points. Assuming 
that the earth can never have had a crust floating on a liquid 
layer of inferior density, computation leads him to 24 million 
years as the maximum period for the time since superficial 
consolidation was effected, provided that the superficial 
temperature gradient and conductivity are correctly determ- 
ined. 
These researches, together with Helmholtz's investiga- 
tion on the age of the solar system, which is incomplete for 
lack of knowledge of the distribution of density in the sun, 
have had a restraining influence on the estimates drawn from 
sedimentation by geologists. IMany and perhaps most 
geologists now regard something less than 100 million years 
