ME. EOBEKT MALLET ON VOLCANIC ENEEGY. 
167 
pressures, produced by the gravitating of the shell itself towards the contracting and 
attracting nucleus beneath it. 
The work thus developed is transformed into heat ; that heat is greatest along those 
lines or planes or places where the movement and pressure together constituting the 
work are greatest. Along or about such places of concentrated compressive and 
crushing work the temperature may locally rise to a red heat, or even to that of fusion 
of the rocky materials crushed, and of the pressing together walls themselves adjacent to 
them. This, then, is the writer’s view of the real nature and origin of volcanic heat as 
now produced in our globe ; it comes not from a free communication with a primordially 
and still fluid interior ocean, nor from such communication with isolated lakes of 
liquid rock, which have no probable existence, but is produced below the places where 
it appears in volcanic vents, or beneath and adjacent to them, by the mechanical energy 
of the compressed-together shell, as that falls down by gravity upon the contracting 
nucleus ; and the heat so produced locally is again consumed locally and disposed of 
by the origination of chemical work and by reconversion into mechanical work, chiefly 
of ejection. 
And thus though volcanic energy as we see it on our globe is not the direct 
product of primordial heat of fusion, it is indirectly due to the loss of that heat, being 
simply one result of the cooling of our globe and of the acknowledged laws of gravi- 
tation. 
Volcanic energy (or vulcanicity in general, as comprehending in it earthquakes and 
other of the so-called plutonic phenomena of geologists) may therefore be defined, 
according to the writer’s view, as follows : — 
Definition. 
67. The heat from which terrestrial volcanic energy is at present derived is produced 
locally within the solid shell of our globe by transformation of the mechanical work of 
compression or of crushing of portions of that shell , which compressions and crushings 
are themselves produced by the more rapid contraction , by cooling , of the hotter material 
of the nucleus beneath that shell , and the consequent more or less free descent of the shell 
by gravitation , the vertical work of which is resolved into tangential pressures and 
motion within the thickness of the shell*. 
68. It has been pointed out that in the earlier stage of secular cooling, when the immense 
contraction was met by deformation of the exterior portions at least of the spheroid, great 
lines of weakness through sharp curved bendings and fractures were produced. All sub- 
sequent action has tended to increase the number and extent of these ; and it will pro- 
* [The production of heat as a consequence of the condensation of gases or vapours in progress towards 
liquefaction or solidification has been noticed by Mr. Herbert Spencer in his “ Essay on the Nebular Hypo- 
thesis” ("Westminster Eeview, July 1858). It need scarcely be remarked that this is altogether different 
from the source of volcanic heat here pointed out, nor has the writer’s view been in any way anticipated by 
Mr. Spencer.] 
