170 
ME. BOBEET MALLET ON VOLCANIC ENEEGY. 
that shell, and in amount is dependent not so much on how much is received from 
below as on the amount of contraction of the material of the nucleus, which is, of 
course, a function of its total loss of heat in all ways. As the heat thus developed 
unequally by compression at different depths may vary, we at once obtain an explanation 
of what has been observed at Dukenfield shaft and elsewhere (namely, discontinuity in 
the series of heat increment), and perceive how a warmer or a colder couche may be inter- 
polated. In fact we have a real source of perturbation distinct from difference of con- 
ductivity and presence of percolated water, which alone engaged Mr. Hopkins’s attention- 
76. But the evolution of heat within the solid shell by the variable compressibility of 
the superposed formations thereof does not end with the compressions of the material 
of each or all the beds. 
Any two superposed beds, such as Q and S' in fig. 6, exposed to the same compressing 
force, as they have unequal coefficients of compressibility, must slide upon each other ? 
and so produce frictional and disintegrative work between the sliding surfaces ; this also 
becomes transformed into heat, and further tends to raise their own temperature and 
those of adjacent beds. 
77. Lastly, we must take into account that the tangential forces cannot be always, as we 
have assumed in f f (figs. 4, 5, 6), uniform at all depths; for independent of any 
general law connecting the gravitation of the shell with those tangential forces (to 
which we shall presently refer), the mere inequality of resistance at various depths, 
which we have shown, must derange that equality of pressure, and even within certain 
limits change its direction locally from being strictly tangential into directions more or 
less oblique, both vertically and horizontally. 
Although we know nothing of the constitution or order of the materials constituting 
even the solid shell of the globe deeper anywhere than perhaps 70 miles at most 
(inferentially), and to perhaps 25 miles by observation and inference, still the discus- 
sions of Durocher and others warrant our assuming it, to a far greater depth than above, 
as not differing greatly from the harder crystalline rocks of the surface, and indicate that 
below the stratified deposits, say below 25 miles or so in depth, the material may be 
presumed, with high probability, to be much more uniform, less shattered, and denser 
than near the surface. 
78. If we have thus discovered a true and a sufficient cause for great local elevation of 
temperature within the solid crust of our globe, it is submitted that we have really 
discovered the origin and nature of volcanic action, and proved it to be only part of the 
acknowledged cosmical machinery of our globe, independently of any question as to how 
hot it was originally, or what length of time may have since elapsed, or what may be its 
internal temperature now (save that the interior of the globe is still hotter than the 
exterior, and that the whole is cooling), or whether the nucleus be liquid or solid, or 
the shell be thicker or thinner. 
For, a sufficient source for the high local temperature at some depth below volcanic 
vents being discovered, the presence at their foci of water, fresh or salt, completes 
