Features of the Earth's Surface. 349. 
The theory above presented, viz: the formation of continents 
and sea-bottoms by unequal thickening of a floating crust, by 
unequal cooling, may, with skillful modification and by adding 
hypothesis to hypothesis, be made to account for most of the 
granite and other igneous rocks do not expand in the act of 
solidifying, but on the contrary, according to the experiments 
of Bischof, notably contract. 
6. The solid shell of the earth, if there be any such, may be 
Proved to be much thicker than is usually supposed by geolo- 
gists of the interior-fluidity school The principal argument 
for the liquidity of the earth beneath a comparatively thin shell 
18 based upon the increasing temperature of the earth as we go 
downward into the interior. “The rate of 1° for every 50 feet 
of descent would give 8000°, the fusing point of iron, at a 
depth of 28 miles.” At this temperature nearly all rocks 
Let s s (fig. 5) be the surface of the earth, and a B depth along 
any radius) Taking A B as an abciss, let the increasing heat 
represented by ordinates. Now, if the density and con- 
ductivity of the earth were constant, then the heat would 
mcrease at uniform rate, and would therefore be correctly 
represented by the straight line cp. At the rate of 1° for 
every 50 feet we would reach the heat ordinate ¢¢ of 8000° and 
the melting point of rocks at the depth of 28 miles But 
