PRINCIPLES UNDERLYING METAMORPHIC PROCESSES — 491 
Transformation points solid-solid are presumably not affected 
by unequal pressure,’ which is equivalent to a shearing stress; in 
this respect they are unlike melting-points. For where melting 
occurs, the pressure will be greater on the solid than on the liquid 
phase, owing to the flowing-away of the latter; at transformation 
points, on the other hand, where both are solid phases, they must 
be equally subject to the pressure, and therefore the comparatively 
large effects produced by unequal pressure on melting-points will 
be absent at inversion points.* Nevertheless it is entirely probable 
that persistent unequal pressure (differential stress) might increase 
the rate of inversion a>, the latter being the form normally stable 
at high temperatures; for it is conceivable that unequal pressure in 
sufficient amount should lower the melting-point of the a-form to 
such a point that an actual progressive melting’ of a takes place 
followed by a crystallization to the 6-form, if that were the form 
stable under these particular conditions. 
To illustrate, there are a number of metastable forms (for 
example, yellow HglI, at temperatures below ca. 120°, yellow PbO 
below ca. 550°) which can be changed into the corresponding stable 
forms merely by scratching or rubbing in a mortar.4. This mode of 
action of unequal pressure recalls that of catalytic agents, and may 
be used to aid in accounting for the fact that metamorphic processes 
occurred easily in those regions which have been exposed to stress. 
Enantiotropic transformation of mix-crystals—We must call 
special attention to a circumstance which is often neglected although 
it is of great importance in petrogenetic systems: namely, that the 
temperature of an enantiotropic inversion has a single definite 
value only when both the forms involved in the equilibrium are 
1 The idea of “‘unequal pressure’’ and its consequences are discussed more fully 
in a later paragraph. 
2 Nevertheless, there might be some comparatively slight effect; for if the process 
of transition occurred through a melting (as is outlined in the next sentence in the 
text), the difference between the effects of such unequal pressure on the melting- 
points of the a and 8 forms would be its net effect in changing the temperature of 
Inversion. 
3 By this it is not meant that the whole of a should at any instant be melted; 
see a later paragraph. 
4It is of course possible that the slight local rise of temperature produced by 
grinding also plays a subsidiary role in this process. 
