180 
ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
The pressures on the two pairs of opposite and vertical faces are equal and opposite 
T and T x . 
The lower face of the cube may be free more or less to descend, by further compres- 
sion of the material far below, whether solid, plastic, or liquid, but the uppermost face 
of the cube is free to ascend by merely compressing and lifting the column of material 
above it ; and as we have shown that the value of T or T, must always and enormously 
exceed that of P, which is the vertical pressure, it follows that if P varies at different 
places in the shell, crushing will take place by vertical deformations and very readily. 
95. Were the shell absolutely homogeneous and isotropic as to its material, still crushing 
must somewhere occur as soon as the pressure there had reached the limit of cohesion. 
But we must, as it appears to the writer, suppose the solid earth shell, however thick, to 
be heterogeneous, and discontinuous or fissured more or less at every depth, until the 
commencement of the plastic or viscous couches that intervene between its interior and 
the nucleus (whether that be liquid or only hot and soft relatively by heat) shall have 
been reached. 
For the Assuring and shattering of the solidified crust, which began when it was very 
thin and was far more extensive and intense as it became thicker, and at a certain 
thickness (when the thermal equilibrium of the whole system had got more nearly to 
what it now is) began to decline, must have gone on, reaching continually further in 
depth as the thickness of the solid crust was increased by accretion to itself of more 
solidified matter at its inferior surface. 
If, then, we admit the highly probable view that the solid crust of the earth at the 
present time is in all its parts more or less heterogeneous, if not in the nature of its 
materials, at least in the physical or molecular state of these at different points, there is 
no difficulty in admitting any amount of crushing and Assuring to be going on within it, 
and such crushing and Assuring must be local and irregular. 
96. We have thus proved that localized crushing of the rocky material of our earth’s 
crust must take place ; and it will not be denied that heat must be produced by trans- 
formation of the work expended in crushing. 
97. But two great questions now require to be answered — namely, how much heat is pro- 
duced by the crushing of a given weight or volume of rock, and whether the total amount 
of crushed rock, or of heat due to it, that we can estimate on admissible data as occur- 
ring in a given time (a year or 1000 years) be adequate to account for the volcanic phe- 
nomena we witness on our earth’s surface, or estimate as necessary thereto. The answers 
to these must be based on experiment. 
98. The mechanical work expended in the deformation or disintegration of a solid must 
all reappear either as heat or as external work of some sort. 
In the case of very inelastic and easily deformed bodies, such as lead, Mr. Hirv has 
shown (Theorie Mecanique de la Chaleur, 2nd edit, part i. p. 58 & c.) that the heat 
