4 Mr. R. Mallet on the Temperature attainable 



the localization ? For example, suppose a horizontal column 

 10 miles in length and 1 in sectional area to be crushed by pres- 

 sure applied at its ends, which of the 10 cubic miles is to be the 

 one 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 

 toed/' 



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 rock 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 of the entire mass of surrounding 

 rock are under the insistent pressure of the superincumbent 

 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 

 crush the cube by its finally giving way, in whatever direction 

 this encastrement by pressure may be least, will be increased over 

 that which would crush it in free air nearly in the ratio in which 

 the imaginary cube is exposed to external pressure greater than 

 that in air. Thus, if the cube of Guernsey granite (No. 12, Table 

 I. Phil. Trans, part 1, 1873, 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 178*3392 lbs. per cubic foot, be supposed situated at a 

 depth often to twenty statute miles, it will require rather more than 

 2*14, or, at twenty miles, 4*28 times as much pressure upon two 

 opposite faces to crush it that it did when in air; and if we assume 

 the displacement of the crushed particles after crushing to be 

 the same as in the case of the cube crushed in air, then the work 

 and the heat due to its transformation will be also 2*14, or 4*28 

 times as great. And as in the case of the cube crushed in air 

 the heat developed was sufficient to fuse (at 2000° Fahr.) 0*108 

 of its own volume, or, in other words, the crushing of 10 

 cubic feet of the rock would be required to raise to that point 

 one cubic foot, then in the case of the imaginary cube situated at 

 the depth of ten miles enough heat would be evolved by the 

 work of crushing each cubic foot to fuse 0*231 cubic foot, or, at 

 twenty miles, to fuse 0*462 cubic foot of the same rock, or nearly 

 half the volume crushed, — and this assuming that the initial 

 temperature of the rock at 10 or 20 miles depth was only 57° 

 Fahr. as in the author's experiments, instead of from 500° to 

 1000° Fahr. or more as it may be at 10 to 20 miles depth. 

 Therefore, under the pressure due to a depth of 20 miles and an 

 initial temperature of 1000° Fahr., the heat developed by the 

 work of crushing each cubic foot of rock will be sufficient to 



