168 
ME. EOBEET MALLET ON VOLCANIC ENEEGY. 
bably not be denied by geologists that every thing we know of points to the existence of 
great sweeping lines of weakness and of broken continuity in the crust of our globe, as, 
for instance, round the shores of the Pacific Ocean. Beneath and adjacent to such 
lines we must suppose the rocky materials fractured and broken up, and over large 
areas and to great depths reduced to discontinuous fragments closely pressed together 
and gripped in contact. 
It is just along and over such places that we find volcanic vents. Along such planes 
of weakness, extending to great but unknown depths, the temperature should be highest 
by the concentration thereabouts of the movements of the compressed crust, as already 
shown. 
69. But such heated areas are not confined to merely vertical planes, at least for so far 
in depth of the entire solid crust as consists of superposed and interposed formations 
differing in mineral character, or (what we are here alone concerned with) in compres- 
sibility and conductivity, one or both. 
70. In fig. 4, if S C represent the depth of the earth’s crust, a portion of which of unit 
thickness is comprised between the vertical planes a Eig. 4. 
and #, and submitted to compression by the opposite 
tangential forces / and /', then if the entire mass. in 
depth between C and S be of homogeneous material 
of uniform compressibility, any rise of temperature 
produced by compression will be uniform throughout 
its mass. 
But if the compressibility be less , say towards C, 
than higher up, then, supposing an adequate force 
acting equally upon all points of the planes/ and/', and that the more compressible 
material is not reachable by the pressures until after the less compressible material shall 
have given way, the temperature will not be uniform, but will be greatest where the work 
of compression is so, and thus we should have variations of temperature with depth in 
any gradual order, whether ascending or descending. 
71. This will also, though in a less regular way, be true if, as in fig. 5, the mass a C b 
include a great plane of weakness, due to fracture &c., 
extending in depth in a variable way. Thus if the 
material be more compacted towards C than higher 
up, and so less compressible, and if, as before, the 
tangential pressures/ and/ act uniformly upon the 
vertical planes a and b, so that they can only approach 
each other at the rate the less compressible matter 
about C gives away, then must the highest tempera- 
ture be developed about C, where there is the largest 
amount of work. 
72. Thirdly, let us suppose the mass a C b to consist of several beds or layers of various 
Eig. 5. 
