Condition of the Interior of the Earth. 
475 
parts of the continent. During the erosion intervals large parts 
of the regions were reduced nearly to sea level by subaerial 
erosion. 
Condition of the Interior of the Earthf —In order to under¬ 
stand how great vertical earth movements may occur, it is nec¬ 
essary to mention modern conclusions as to the condition of the 
interior of the earth. It is certain that the material deep down 
is highly heated. Most lines of calculation indicate that the 
temperature near the center of the earth must be several thousands 
and may be many thousands of degrees. Such temperatures as 
probably exist at the earth’s center would at the surface make 
the most refractory rock as liquid as water, if indeed it did not 
vaporize it. However, it does not follow that under the tre¬ 
mendous pressures deep within the earth the material is gaseous 
or even liquid. It is a well known law of physics that bodies 
which contract on solidifying may be held in a solid condition 
by great pressure at temperatures which would render them liq¬ 
uid if under less pressure. It has been concluded by some physic¬ 
ists that the pressures within the earth are so great that even at 
the high temperatures calculated the material is held in the solid 
condition. Deductions based upon the tide-producing force of the 
sun and moon show beyond all question that the earth has an ex¬ 
ceedingly high rigidity when subjected to great and not long con¬ 
tinued stress. As Kelvin states it, the earth “Is not, as com¬ 
monly supposed, all liquid within a thin solid crust of from 30 
to 100 miles thick, but that it is on the whole more rigid cer¬ 
tainly than a continuous solid globe of glass of the same diam¬ 
eter, and probably than one of steel. ” 2 While this conclusion 
is not doubted by geologists if it be confined to the rigidity 
shown by the earth to the daily tidal stress, their observations 
lead them to believe that the earth shows real plasticity when 
subjected to long-continued, moderate stresses. Experiments 
upon viscous wax show that under pressure it becomes highly 
^he mathematical theories of the earth, by R. S. Woodward: Am. 
Journ. Sci., 3rd ser., Vol. 38, 1889, pp. 337-355. 
9 Treatise on natural philosophy, by Thompson and Tait: Part II, 1890, 
p. 485. See also Popular lectures and addresses, by Lord Kelvin: Vol. 
II, 1891, p. 306, and Vol. Ill, 1891, pp. 189-190. 
