300 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
strained within the elastic limit or granulated, and with the last the 
former occurs, for even where the original mineral particles are broken 
the individual granules ordinarily show strain-shadows. 
The question of the relation of a state of strain in minerals to the po- 
tentialized energy here naturally arises. Barus* has shown in the case 
of strained metals that the work done in straining them is very largely 
potentialized. In ‘“ glass-hard ” steel, strained to the point of rupture, 
50 per cent of the energy used was potentialized ; in brass, 40 per cent; 
in copper, 25 per cent. A larger percentage of the energy was potential- 
ized in the earlier stages of strain than in the later stages. The explo- 
sive action of a Prince Rupert drop, when a point is broken, shows that 
a large amount of energy is potentialized, or that the glass is in a high 
state of strain. 
The experiments of Barus and the condition of the Rupert drops show 
that in strained minerals energy is probably potentialized. The fact 
that glass releases itself from strain by crystallization, as noted (page 
289), may be taken as evidence that minerals in a similar condition are 
more readily rearranged by the chemical forces through solution than 
they would be in an unstrained condition. In an interior state of strain 
we therefore have a cause for recrystallization. This cause, high tem- 
perature, and unequal distribution of exterior pressures—which always 
must be a condition in rocks because of the unequal strength of the 
minerals—appear to be sufficient reasons for recrystallization in rocks 
which have been pro‘oundly affected by mass dynamic action. Slight 
movement disturbs the equilibrium. As soon as a state of strain is pro- 
duced the processes of solution and recrystallization set to work to adjust 
the minerals. 
The process in many respects is analogous to the very rapid solution 
and crystallization of glass described by Barus (see pages 289, 290). It 
may be recalled that 180° C. is a critical temperature in the crystalliza- 
tion of glass. At temperatures as high or higher than this, which un- 
doubtedly prevail in the deepest-seated zone of deformation, the process 
can go on with comparative rapidity. At any moment the substances 
are present almost wholly as minerals. However, superheated water 
is in the capillary and subcapillary spaces between the particles, and 
through this as a medium adjustment by solution and recrystallization 
goes on continuously during the deformation. At any given moment 
only an exceedingly small part of the material is in solution ; but under 
the kinetic theory of solutions all materials in a state of strain, or sub- 
ject to unequal pressure, or not in a compact state, will be more ready 
* The mechanism of solid viscosity, by C. Barus: Bull. U. S. Geol. Survey, no. 94, 1892, pp. 
107, 108. 
