1895. ] NEW YORK ACADEMY OF SCIENCES. 73 
It is important to appreciate that the strata rest upon one an- 
other substantially as if fluid, because the arch of the crust is so 
flat. The compressive stress on any portion considered as a 
keystone is 30 times the crushing strength of steel, and 500- 
1000 times that of granite and limestone, whence it follows that 
the earth is practically in hydrostatic equilibrium. It also fol- 
lows that the pressures in the interior are excessive, and that at 
the center the pressure is about 3,000,000 atmospheres. The 
earth is ‘solid,’ as the word is used by Lord Kelvin, that is, it 
has no cavities below a comparatively shallow depth. The ex- 
planations of the changes of latitude lately advanced and based 
on internal hollows in which loose matter rolls around are absurd. 
There is perfect continuity of matter, and there is only fluidity 
when for some local cause the pressure is somewhat relieved. 
As Major Dutton has shown, the transmission of vibrations from 
the centrum of the Charleston earthquake indicated a medium 
nearly as homogeneous as steel. 
Geologists have had to account for movements of the crust, 
such as subsidence, elevation, crumpling, folding, etc. Two ele- 
mentary forces are necessarily appealed to. The first is Gravity ; 
the second that due to the Harth’s Internal Heat. The idea of 
the earlier geologists that the earth cooled and contracted and 
hence caused the disturbances has been mostly relied on as an 
explanation, but for the last ten or fifteen years has been felt to 
be insufficient. The idea of Babbage and Herschel that loaded 
areas, or areas of sedimentation sink and crumple up the adja- 
cent areas as mountains, tending thus to renew and perpetuate 
regions of upheaval, has also had believers. This has had its 
best formulation in the recent doctrine called isostasy, which re- 
gards the earth as a body in essentially hydrostatic equilibrium, 
and as balancing inequalities of pressure by subterranean flow. 
The speaker regarded this doctrine, however, as insufficient in 
that it furnishes no start and tends to run rapidly down. We 
need secular contraction to keep isostasy at work. The earth’s 
internal heat is the great store of energy available for this 
purpose. How to explain the earth’s internal heat is a hard and 
dark problem. The nebular hypothesis, first outlined in Leib- 
