172 
ME. EOBEET MALLET ON VOLCANIC ENERGY. 
system. The change was gradual ; and just as the epochs of land and sea forming by 
deformation overlapped that of mountain building by crumpling, and that again has 
overlapped in time our existing epoch of volcanic crushing and explosive action, so did 
the great epoch of hydrostatic igneous action overlap more or less the commencement 
of the existing volcanic era. The more ancient form of igneous action, by which the 
enormous trap protrusions &c. were poured forth upon a scale, as lately observed in 
California, wholly inconsistent with existing volcanic forces, continued in force down 
to comparatively recent periods, as of the Chalk, and may even yet be going on possibly 
under the sea. But two main characteristic facts remain — namely, that the most 
ancient igneous workings were hydrostatic and not explosive, while the existing or 
volcanic activity (properly so called) is explosive and is not hydrostatic ; and, secondly, 
that upon the whole this last or existing form of igneous action (the explosive) does 
not, when viewed largely, date back further than to some part of the Secondary 
period, and that a preponderant amount of it is of still more recent date.] 
80. It follows, therefore, that a like machinery of volcanic action to that now existing in 
our globe cannot have existed in any other planet anterior to its surface having 
assumed such thermometric conditions as enabled water or some equivalent fluid to 
become permanent upon its surface. That temperature might be very different from 
ours as now existing, and was once no doubt far above 212° upon our globe. 
But it does not follow (as has, indeed, already been suggested) that in a planet or 
satellite constituted very differently from our globe volcanic action may not be 
maintained, for a longer or shorter time, by chemical actions, or by these and mechanical 
ones together, of a far different nature from the vulcanicity seen upon our globe — such 
as the evolution of gases from liquid or solid matter at one temperature and their 
absorption at others, as in the case of melted silver and copper absorbing oxygen, or 
of the numerous cases of such chemical actions in compounds discovered by Tessie 
du Motay, or in many other imaginable ways. 
81. The writer is now called upon to show that, assuming the origin for the heat thus 
produced, which is the moving energy of the volcano, the conditions are such as to prove 
it to follow from forces real and adequate to the result. 
He proceeds to do so, and for this it is necessary to show : — 
1st. That the gravitation of the unsupported or but partially supported solid shell 
is adequate to crush into powder all the materials of which it consists, and that no 
matter how thick the shell may be unless equal to the whole radius. 
2nd. What is the total amount of contraction of materials analogous to the 
rocks of the solid shell, between their temperature of fusion or one above it and 
that of our atmosphere now. 
3rd. What is the mean work per unit of weight and volume necessarily expended 
in crushing to powder the rocks of which the solid shell consists, and what is the 
amount of heat due to the transformation of such work. 
