ME, ROBERT MALLET ON VOLCANIC ENERGY. 
169 
compressibilities and submitted to the forces f and f as before ; and, further, suppose 
that, for example, the bed Q (fig. 6) is less compres- 
sible than at S' just below it, or than other beds in the 
series. Then, under the conditions, as the work of 
compression is greatest in the bed Q, that bed will, 
if its compressibility be everywhere alike, be uni- 
formly hotter than the adjacent beds. 
There will here be not a vertical or nearly ver- 
tical plane or plate of increased temperature, but a 
horizontal one. 
73. The greatest amount of work may not be always in the least compressible rock ; for as 
work is the product of pressure and motion, a soft easily yielding rock with a large range 
of compressibility may, by the application of a given amount of pressure, produce the 
greater amount of work. 
74. The extent to which a bed of rock may thus be heated by compression without any 
crushing , i. e. by pressure far within its elastic limits, may be illustrated by an example. 
In the writer’s own experiments on the compressibility of the rocks at Holyhead 
(Appendix to Account of the Earthquake Wave-Experiments made there, Phil. Trans. 
1862, vol. clii. pp. 663-676), he found that certain quartz rocks and certain slate 
rocks both bore about 12,000 lbs. per square inch before their elastic limits were passed, 
the total compression at this pressure being 0T3248 for the quartz and only 0-04464 
for the slate upon the unit length. The foot-pounds of work in compressing each .to 
this amount (from which the rock would recover when released), divided by J (Joule’s 
equivalent), shows that a prism of a foot square and 100 feet long of each rock would 
develop the following British units of heat : — 
Quartz = 295-200. 
Slate = 100-800. 
If consisting of these rocks, therefore, and both compressed by precisely the same force, 
12,000 lbs. per square inch, the bed Q (fig. 6) of quartz would be nearly three times 
as hot as that, S', of slate beneath it, and the former would communicate its heat to the 
beds both above and below. 
75. In this we see, then, one very sufficient cause for great inequalities in the rate of 
increments of hypogeal temperature, which, so far as the writer knows, has escaped 
notice, and which did not occur to Mr. Hopkins when (see his paper, Phil. Trans, 
vol. cxlvii., 1857, alluded to in a previous part of this paper) he discovered that central 
heat and difference of conductivity alone were not sufficient to account for the phenomena 
of increase of heat with depth. Indeed, as we shall see before concluding, the pertur- 
bations of the hypogeal temperature arising thus from intestine pressures and motions 
within the earth’s solid shell must be far greater than has hitherto been suspected, and 
may amount to a very large fraction of the heat received from the nucleus. 
The heat thus produced by such intestine work is, so to say, actually generated within 
Fig. 6. 
