1 877. [ On Underground temperature. 187 
It is almost universally believed that the inequalities of the 
earth’s surface were produced, and are maintained against 
degradation, by the contraction of its volume. The writer 
some time ago made a calculation* of the magnitude which 
these inequalities might be expeCted to have assumed, on 
Sir W. Thomson’s view, expecting fully that a cooling solid 
earth would account for them all, and for losses by denuda- 
tion besides ; but to his surprise they came out far smaller 
than those actually existing. The elevated traCts and the 
ocean bottoms present deviations from a spherical surface 
much greater than could arise from a cooling solid sphere. 
Capt. Dutton, an American geologist and physicist, was led, 
by a similar consideration, to abandon the contraction theory 
altogether, and to seek for some other explanation of the 
formation of the inequalities. In short, it seems clear that 
if Sir William Thomson’s views upon this point are correct, 
then the inequalities of the earth’s surface are not due to the 
contraction of the sphere, and the plications of the strata 
must be accounted for in some inexplicable way. If, on the 
other hand, the inequalities of the surface are due to con- 
traction, there is strong reason to doubt that the earth 
became solid throughout, and that it is now cooling as a 
solid, according to Sir W. Thomson’s law. 
There is another faCt connected with mountain chains 
which has an important bearing upon this question. To 
adopt Prof. Green’s description of these : — “ They owe their 
superior elevation to the faCt that the rocks of which they 
are composed have been squeezed and ridged up to a greater 
height than the rocks of the country on either side.”! Now 
this peculiarity, as was pointed out by Capt. Dutton, and 
also implied by the writerf in his argument against Mr. 
Mallet’s theory of volcanic energy, necessitates a “ slip ” of 
the earth’s superficial strata over whatever it be that under- 
lies them. It is obvious that a more or less perfectly fluid 
substratum is requisite to account for this peculiar character 
of mountain-chains. What this fluid condition may be, 
whether permanent or transient, whether it is connected 
with volcanic aCtion or not, are questions open to discussion. 
But there must be-— if not permanently, at least during 
epochs of the mountains rising — some such condition pre- 
sent. Indeed the nearest analogy to the formation of a 
mountain-chain seems to be found in the crushing together 
of the adjacent edges of two sheets of floating ice, the 
flotation of the sheets being a necessary part of the 
* Cambridge Phil. Transactions, 1873. 
f Geology for Students and General Readers, p. 462. 
+ Phil. Mag., October, 1875, p. 9. 
