THE GENERAL FEATURES OF THE EARTH 21 
pierced into the crust, it has been found that the tem- 
perature increases as the depth becomes greater. There 
is much variability, but everywhere this is true, and 
the average rate of increase is one degree for every fifty 
or sixty feet of descent. In one valuable gold and 
silver mine, the temperature became so high that it 
was necessary to abandon it. 
If this rate of increase in temperature continues, the 
interior of the earth must be imtensely hot; and, in 
proceeding below the crust, a region must soon be 
reached where the temperature is high enough to melt 
the rocks. But great pressure raises the melting-point. 
That part of the earth which lies deep beneath the 
crust, is weighted down with a tremendous load of 
upper rocks, with a pressure perhaps equal to hundreds 
or thousands of tons to the square inch. Therefore, 
under this pressure, their melting-point is raised to a 
point higher than is necessary to melt them at the sur- 
face. So it is believed that while the heat is extreme, 
the deep-lymg rocks are prevented from melting by the 
pressure of the crust. 
There seems to be sufficient reason for the belief 
that the earth is hot within, and, like the sun, contin- 
ually cooling. Loss of heat necessarily means a loss 
of bulk or shrinkage on the part of the rocks which 
are cooling, and it is thought that this contraction 
causes the constant movement of the crust seen in 
