THE ERUPTION OP ETNA. 
449 
necessity of a vast amount of heat for converting the mineral 
lavas into fluidity ; have been all regarded as strong presump- 
tions in favour of a universal substratum of the volcanic 
fires beneath the solid crust of the earth. 
For myself (I may be rash)^ I do not disguise my heretical 
disbelief in the still favourite geological hypothesis of a molten 
core existing within our globe. The contraction of this outer 
crust of our planet, as it slowly parts with heat into space, 
upon this liquid core, is by many, if not by most modern geo- 
logists, regarded as not only the main means of the uplifts 
and depressions of continental lands, but also as the source 
of earthquakes and volcanos. Attempts even have been made 
to associate the latter phenomena with tidal actions produced 
on this internal molten ocean by lunar influence, as also with 
alterations in the conditions and barometrical weight of the 
atmosphere. 
Nor are we digressing to this topic without a purpose. 
There are but these two main theories for accounting for the 
occurrence of volcanic eruptions — ^the chemical and the me- 
chanical theories, or that founded on the belief that chemical 
energy 'can give rise to sufficient heat by its action upon 
certain constituents of the rock-masses forming the substance 
of our earth, and that based upon the supposition of a pre- 
existing universal subterranean source of heat, and which 
hence regards all volcanic phenomena as the mechanical effects 
of this grand central furnace. Now, it is fair play to speculate 
either upon or against this terribly hot infernal region^'’ upon 
which the mechanical theory is based, because nobody really 
knows anything about it, and nobody is ever likely to go so 
far down below to see ; and it is also well to state a fact or 
two against it, because it has had too much prominence, and 
stands not only greatly but unjustly in the way of a fair con- 
sideration of the chemical theory. The most vaunted proof 
of the existence of an internal molten core within our globe 
has been the assertion that in deep mines and borings there is 
found an increase of temperature in the rock-masses of oue 
degree of temperature for every 60 feet of depth. There is 
reason for doubt on this subject; but assuming the data as 
our modern Plutonists give them, namely, that this world has 
consolidated by cooling an outer solid sphere or crust 
800 miles in thickness, around an interior still liquid nucleus, 
and that as we mine or bore deeper into this solid crust the 
temperature of the rock-masses does increase at the rate stated ; 
then at 164 miles, only one-fifth of its thickness down in its 
outer or coolest part, we should have a temperature equal to 
that of the oxy hydrogen flame, or 14,580° F. 
Dr. Miller has shown by the spectral analysis of sunlight 
