20 ELEMENTARY GEOLOGY 
us from outside the earth. The tides, caused by the 
attraction of moon and sun, furnish an illustration of 
the effects of this energy; and the winds and rains, 
the alternations of temperature, the waves and cur- 
rents of the ocean, and many of the other agents 
which are operating upon the face of the earth, are 
other illustrations. 
But there is another class of changes which cannot 
be traced outside of the earth. Why do volcanoes cast 
out rock, ash, and lava; and why are mountains rising 
and the continents changing in elevation? These feat- 
ures are surely to be traced to some condition within 
the earth; and we believe that, in some way, they are 
dependent upon great stores of heat which exist beneath 
the surface. 
It was formerly thought that the earth consisted of 
a rind or crust of slight thickness, and that within this, 
there was a fiery, molten sphere. Astronomers and 
physicists have assured us that such a condition is 
impossible, and that the earth thoughout its mass is as 
rigid as steel. The proof of this seems convincing, and 
is accepted by most geologists, although the word crust, 
now with a different sense, is still used in speaking 
of the outer portion of the earth. 
In many places, however, volcanoes bring molten rock 
to the surface from a considerable depth; and every- 
where on the globe, where deep mines and wells have 
