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314 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. ° 
The energy for rock deformation may be divided into two parts— 
energy for mechanical work and energy for chemical work. The me- 
chanical work is of three kinds—(1) the subdivision of the rocks, (2) the 
transfer of the material in order to produce a changed form, and (3) the 
friction between the parts of the subdivided rocks during the transfer. 
The most useful comparison as to the amount of energy spent in the 
three zones is upon the basis of average mass deformation. I mean by 
average mass deformations the strains necessary to change the shape of 
unit masses of rock in a nearly similar way, so that the exterior forms are 
practically the same. To illustrate: A cubic foot of rock may be sup- 
posed to be divided into 10 horizontal slices and sheared parallel to 
these slices, so as to produce, ignoring the minor corners, a roughly 
rhomboidal mass (figure 1). If instead of 10 slices there were 100 slices, 
the approximation to a rhomboidal mass would be closer; if a thousand, 
closer still, and so on, until the slices became of infinitesimal thickness, 
when the mass would be rhomboidal. In all of these strains the mass - 
- deformation averages about the same. 
It is perfectly clear that in the case of this hypothetical deformation 
the amount of work in rupturing would be directly as the number of 
slices. The average mass deformation is substantially the same, and the 
energy required for change in form—or, in other words, for transfer of 
Fieaure 1.—Iilustrating average mass deformation 
material—is nearly constant. The total amount of differential move- 
ment or shear is practically the same in all cases, and therefore the fric- 
tion is nearly constant. Hence in the case of the illustration the energy 
for the deformation is almost directly as the number of slices ; but in the 
case of the crust of the earth, supposing the fracturing to become closer 
as depth increased, the energy required for a given mass deformation 
would increase with depth for two reasons: (1) More energy is required 
for the finer subdivision, and (2) the load increases with depth; and 
therefore the energy required to overcome friction also increases with 
depth. The energy required for the similar transfers of material remains 
practically the same at all depths. 
It has already been seen that near the surface the dominant deforma- 
tions are relatively wide-spaced joints and faults ; that with increase of 
