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the part of this attracting force to decrease in intensity, 
becomes a fair question ? If there be, then its action upon 
these dense and central portions would cause an expansion 
in the mass, which expansion would cause the elevation 
of the surface, and the forcing up of any matter which 
might be in a fluid state above the expanding mass. "We 
know as a fact that heat will cause an expansion in almost 
all substances, that is, that heat will cause the particles to 
separate in a measure from each other, the amount of 
separation depending on a law, the details of which are 
unknown. Thus, if there be any such tendency on the 
part of the lower strata, we have an adequate cause for 
the expansions mentioned above. 
Let us now consider another question. Heat produces 
expansion in almost all substances : might not the exposure 
of the earth to great heat cause a considerable elevation 
of land by means of the expanded strata ? Certainly this 
result might follow ; and if we could see a means by which 
an additional amount of heat might be obtained by the 
earth, we should not be supposing a cause which was itself 
inexplicable. We must at this point entirely separate our 
ideas from that vague indefinite cause spoken of as the 
central heat theory, which assumes that the earth at a short 
distance beneath the surface is in a state of fusion from 
intense heat. There is not a well established fact of any 
kind to prove this assumption, and there are very few facts 
in geology which are explained even if we grant the truth 
of the theory. This theory about the centre of the earth 
being in a state of fusion from heat is based upon the 
fact that if we descend into the earth we find a greater 
heat than upon the surface. Surely this is a very slight 
foundation upon which to build so singular a supposition. 
Yet this theory is assumed by many individuals to be so 
firmly established by facts as to be unquestionable. 
