R. Mallet-—Temperature attainable by Rock-crushing. 259 
detrital masses under great pressure Mallet’s figures of course 
offer no measure whatsoever; nor is this, or even the thermal 
coefficients resulting from his rock-crushing experiments, a 
all necessary to the establishment of the postulates of his 
theory.” 
Subsequently the Rev. O. Fisher, in a paper read before the 
Geological Society of London, May 12, 1875, entitled “Re- 
marks upon Mr. Mallet’s Theory of Volcanic Energy,” has 
repeated the observations of Professor Hilgard, and extended 
his objections to the author’s theory in general in a way which 
appears not warranted. It will be sufficient here to quote the 
following from Mr. Fisher's paper :—“ Indeed the form in which 
the objection to Mr. Mallet’s reasoning suggested itself to my 
mind on first reading his paper was simply this. If crushing 
the rocks can induce fusion, then the cubes experimented upou 
ought to have been fused in the crushing; and I still adhere 
to this simple mode of expressing my objection.” Again :— 
“He considers that the heat so developed may be localized, 
and that the heat so developed by crushing, say 10 cubic miles of 
rock, may fuse 1 cubic mile. But I ask why so} e work is 
equally distributed throughout; why should not the least be 
so also? Or if not, what determines the localization? For 
example, suppose a horizontal column 10 miles in length and 1 
in sectional area to be crushed by pressure applied at its ends, 
which of the 10 cubic miles is the one to be fused? But if no 
cause can assign one more than another, it is clear that they 
will all be heated by 170° and none of them fused.” 
If a cube of rock, which in free air is found to crush under a 
certain pressure, be imagined situated deep within a mass of 
similar rocks and there crushed, it does not admit of dispute 
that the work necessary to effect crushing must be largely 
increased ; the particles of the cube and the entire mass of sur- 
rounding rocks are under the insistent pressure of the il ed 
cumbent rock in a state of elastic equilibrium. It follows, 
therefore, that the pressures of the surrounding rock produce 
the same effect upon the cube as regards resistance to crushing 
as if they were cohesive forces acting within the cube; and the 
work necessary to erush the cube by its finally giving ray in 
i i su t, 
nearly in the ratio in which the imaginary cube is ex to 
external pressure greater than that in air. Thus, if the cube of 
Guernsey granite (No. 12, Table I, Phil. Trans. part 1, 1878, 
p- 186) which required 4,336,712 lbs. per square foot to crush 
It in air, equivalent to a superincumbent column of the same 
rock of the mean specific gravity 2°858, or weighing 1783392 
Ibs. per cubic foot, be supposed situated at a depth of ten to 
