42 Interior Condition of the Earth. — Le Conte. 
the density of the earth and therefore the increase of pressure 
were uniform. But the earth undoubtedly increases in density 
with depth. Therefore the pressure and therefore also the 
elevation of fusing point increases at an increasing rate. 
Therefore the line of fusion is a line concave outwards 
(fig. 3, fg). 
The probability of touch- 
ing and even cutting and 
again leaving is thus great- 
by increased, and the possi- 
bility of cutting higher up 
and still escaping is also in- 
creased. Thus the probabil- 
ity of the existence of a sub- 
crust layer would be in- 
creased, its position would 
be nearer the surface and its 
thickness would be less on 
this view than on the supposi- 
Sition of the line of fusion Fig.3. ce=lme of actual temperature,/ ?= 
1 . , . T . T line effusion. 
being a straight line. 
But there is still another principle not yet taken into ac- 
count. We have thus far taken the fusing point at 2,500°. 
This is about the fusing point of rocks under dry heat. But 
in the presence of water and in proportion to its percentage 
fusion of rocks will take place at a much lower temperature. 
If the percentage of water be sufficient it may take place at 
800°-l,000° F. To distinguish it from true dry igneous fusion 
this has been called aqueo-igneous or hydrothermal fusion. 
Now, water is found at all depths to which we have penetrated 
the earth. All lavas also contain water combined with the 
fused magma in greater or less quantity. If by this consider- 
ation we reduce the temperature of fusion we should thereby 
render the existence of a sub-crust liquid still more certain 
and bring its position still nearer the surface. 
It seems almost certain therefore that the interior constitu- 
tion of the earth is that of a substantially solid globe ; but 
with a sub-crust liquid layer separating a solid nucleus from a 
solid shell ; and that the layer is not very many miles below 
the surface nor is it of any great thickness. 
The manner in which the sub-crust layer was formed was 
