PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 591 
sidered as a single system, within which complete mutual actions 
and reactions are possible. This may well be so if the original 
rock was fairly homogeneous and not too coarse grained. In the 
case of rocks the solubility of most of the substances concerned is 
so small that contact with a small amount of material can suffice 
for the production of saturated solutions; and so the new material 
will be the same throughout such a system, although the amount 
of material in solution at any moment is small. But whether all 
of the original components, which by reason of the changed condi- 
tions have become unstable, disappear or not depends upon the 
rate of reaction, upon the time, and upon the presence of catalytic 
agencies; furthermore, if any of the components or intermediate 
or reaction-products are present as grains of appreciable size, they 
may become coated with some stable substance and hence—owing 
to the slowness (and even absence) of diffusion in the solid state— 
be protected against further action. 
When to these circumstances which hinder the attainment of 
equilibrium we add the complications introduced by phenomena 
such as resorption,’ the occurrence of reactions in the solid state 
(including*monotropic and enantiotropic changes) and the exist- 
ence of compounds which, while unstable on fusion, are able to form 
at lower temperatures; further, when we consider that some of the 
essential components may by reason of their volatility have 
escaped from the system and that the influence of pressure upon 
stability relations is practically unknown—when we remember 
this formidable array of possibilities, we should hardly expect to 
draw immediate or final conclusions merely through application 
of the phase rule in such complicated polycomponent systems as 
rocks. 
Consequently, the phase rule cannot always be expected to 
hold rigidly or to serve in any manner as an ‘“‘open sesame” in such 
studies. What we observe is that there appear, for a given meta- 
morphic process and for a given number of components, only a 
limited number out of all the possible compounds; moreover, that 
the number of definite and important minerals formed by a meta- 
morphic process usually does not exceed the maximum number 
' For an example, see p. 593. 
