476 
Van Else—Earth Movements. 
rigid. 1 Molasses candy when subjected to the sudden stress of 
a blow is as brittle as glass. Under slight but long continued 
pressure it is readily deformed without fracture. We know so 
little about the state of matter at the temperatures and pres¬ 
sures, over 3,000,000 atmospheres, 2 3 that must exist at the center 
of the earth that we could not assert that a viscous liquid 
would not there be as rigid as glass or steel when subjected to 
stress of brief duration, and thus the conclusion be reached that, 
the liquid is a solid. But it would not be wise to apply either 
of the terms solid or liquid to it. We know what these terms 
mean in reference to matter under temperatures and pressures 
obtainable by experiment, but to apply them in the same sense 
to the material deep within the earth, which is subject to pres¬ 
sures and temperatures far beyond our experience, is certain to 
lead to misconception. 
It is believed that the material of the interior of the earth 
is so highly heated that it would be liquid or gaseous at the 
surface, and that it is very rigid when subjected to stresses 
of short duration, but is plastic under long-continued, moderate 
stresses. Whether this material exists as crystallized minerals, 
or as highly viscous amorphous substance, or in some state of 
matter of which we have no knowledge whatever, or of a combi¬ 
nation of these, cannot be asserted. 
Contraction of the Earth .—-How vast and varied are earth 
movements has to some extent been stated. The main cause 
ordinarily assigned for these movements is a more rapid con¬ 
traction of the interior of the earth than of its exterior shell. 
As a result the outer part is deformed in such a way as to bring 
its too large outer part into adjustment with the inner part. 
The chief, and ordinarily the only cause given for such contrac¬ 
tion is the loss of heat due to secular cooling. In another place a 
I discuss the various other important causes for contraction, 
1 The flow of solids, by William Hallock: Phil. Soc. of Washington, 
Vol. XI, 1888-91, pp. 509-511. 
’Physics of the earth’s crust, by Osmond Fisher: London, 1881, p. 33. 
3 Estimates of crustal shortening, and causes for the same, by C. R. 
Van Hise: Journ. of Geol., Vol. VI, Jan-Feb., 1898. 
