MR. ROBERT MALLET ON VOLCANIC ENERGY. 
°21 
inexplicable, and introduces no consequences in themselves either inexplicable or con- 
trary to observed facts, but all parts of the theory tit together with the facts so far 
as we observe them in nature, like the parts of a “ dissected map ” from which no one 
piece can be left out nor to it any new one added, then the writer submits that, on true 
principles of philosophy, the theory may be regarded as a true interpretation of Nature. 
[Note . — The views of Babbage and of Sir J. Heeschel as to the nature of volcanic heat have been alluded to 
at p. 158. It would have been difficult, however, before the development of the principles contained in the 
preceding paper to have shown the insufficiency of those views. 
As they still linger in the minds of some geologists it seems desirable to advert to them here. Herschel’s 
views are found, as stated by himself in letters to Sir Ciiaeles Lyell and Sir Roderick Murchisoh, in vol. ii. of 
the Proceedings of the Geological Society of London. 
Briefly they may be said to embrace two distinct heads : 1st, subversions of mechanical equilibrium by sedi- 
mentary deposits unequally laid upon an extremely thin solid crust floating on a liquid nucleus ; 2ndly, the 
consequences of such sedimentary deposits in causing a local rise in the geothermal couches. 
The former we may pass, the existence of any such thin crust being inadmissible. As regards the latter it 
is enough to say that it fails to afford any adequate supply of heat sufficient to account for terrestrial vulcanicity. 
The gradual rise of an isothermal couche in the way suggested by Heeschel may possibly afford an adequate 
supply of heat to account for the phenomena of metamorphism, or more properly pyromorphism, but not for the 
play of any ejective volcano. Space forbids any proof of this here; but the nature of the objection may be 
illustrated by an example. 
If we suppose, what certainly does not exist anywhere on our globe, that sedimentary deposits are going on 
continuously over some large area at the rate of 50 feet in depth per annum, it will require above 105 years to 
deposit a mile in depth ; if we further suppose the conductivity of the superficial sediment the same as of that 
already deposited and deeply buried, the annual rise of any given geothermal couche beneath a given square mile 
of surface will be 50 feet, and the increase of temperature of the material just above the former level of the 
couche will be about 1 degree. The heat added, therefore, will be but one degree or thereabouts in a mass of 
sediment 50 feet in thickness and a square mile in area. This amount converted into units of heat is there- 
fore the entire magazine of heat to supply the annual volcanic work per square mile, and, even on this extreme 
supposition as to rate of deposit, would prove wholly inadequate for the fusing and lifting work and waste required 
for the phenomena of active vulcanicity as now existing. 
On principles derived from the above it may further be shown that making the very largest admissible esti- 
mate as to the total amount of sedimentary matter deposited annually over the entire globe, that amount would 
be altogether inadequate, upon Heeschel’s views, to supply the heat required for the annual vulcanicity of our 
globe as estimated in the preceding pages. 
Herschel’s views were apparently hastily struck off, and so far as they have been accepted by American and 
other geologists, have been so apparently on the credit due to the reputation of their author. 
Had thermodynamics, however, been sufficiently advanced in Heeschel’s lifetime to have enabled him to test 
his views by their aid, there can be no doubt but that that illustrious man would have himself discerned the 
untenability of his theory. 
In conclusion it may be added that the notion of some geologists that no supply of surface-water is necessary 
to existent volcanic action, but that the supply of water is derived from that liquid preexisting in vesicular 
cavities of the deep-seated rocky material, maybe shown to present no adequate supply for the volume of steam 
in relation to the solid and liquid ejecta as seen at volcanic vents. — July 1S73.] 
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