ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
177 
yet quite plastic, like a viscous liquid, to the movements of which, however, it may much 
approximate. The superior surface is the most rigid as to conditions due to tempera- 
ture, hut it is for miles in depth shattered, heterogeneous, and more or less discontinuous ; 
the resistance , therefore, at the surface is smaller than at some considerable depth 
beneath. 
Again, as we must suppose (to accord with almost any conception we can form of the 
change produced by hypogeal heat) that the lower surface of the shell is softer and 
more viscous than its upper surface, inasmuch as at the former it passes into a much 
hotter and probably liquid nucleus below it, so the thrust T (whatever its effect may be to 
squeeze and distort a more or less compressible viscous mass at the lower surface of 
the shell and just below it) must produce its greatest mechanical effect in dislocation and 
crushing at some point above. The mere squeeze of a viscous mass, producing com- 
pression or distortion or both, must produce transformation of work into heat, and so 
may reduce to a fused liquid state matter before viscous only ; and in so far as this can 
act in the way of cause, volcanic activity must not be excluded from the local com- 
pressive actions, paramount amongst which is their effect in crushing, as being all 
manifestations of the tangential pressures within the crust to which volcanic action is 
here ascribed. 
88. [It will be lower down from the earth’s surface as the uppermost formations are 
less resistant, and higher above the lower surface where the materials pass from the 
solid to the liquid condition as the depth occupied by viscous matter is greater. It 
will also vary with the - depth, in so far as the coefficients of density, ridigity, and ulti- 
mate cohesion are themselves affected by depth and pressure. 
With a given thickness of solid shell the problem of the deptli at which the volcanic 
couche should be found is at present, therefore, not determinable, though we may 
make many more or less probable hypotheses as to the constitution of the crust, from 
which probable depths for it may be inferred — as, for instance, if we assume a certain 
depth below the surface at which the material is homogeneous and unshuttered, and that 
the coefficient of viscosity decreases according to some assigned law from a liquid inferior 
surface until we ascend to the above level.] 
In these considerations (which the writer is quite conscious are very far from forming 
a mathematical investigation of an exact character, or embracing any thing like the 
whole of the complicated relations that are presented in nature) it is assumed that each 
spherical element of the entire thickness of the shell supports itself, and that the value 
of P is not largely affected by the diminution of the force of gravity in descending, 
no attempt being made to follow out the complex conditions that may arise when 
the crushing has taken place in various conceivable ways. 
89. The writer’s immediate object here is limited to proving that the resolved forces of 
gravitation are sufficient to produce crushing of the solid terrestrial shell, whether that 
be thick or thin, if left partially unsupported by the shrinking away of a cooling and 
contracting nucleus from beneath it ; and this he believes he has now done. 
