590 JOHN JOHNSTON AND PAUL NIGGLI 
a component is so insoluble that its rate of reaction seems so 
small as to be negligible does not prove that in geological phenomena, 
for which unlimited time is available, the influence of such a com- 
ponent can be neglected. Moreover, the similarity in chemical 
nature of certain oxides does not justify grouping them together 
as a single component, in an effort to simplify the system under 
consideration; for the behavior of the numerous two-component 
systems hitherto studied shows that the grouping-together of the 
oxides of the alkalies, or of the alkaline earths, or of alumina and 
iron, is of doubtful validity. 
By means of the microscope it is often possible to distinguish 
between original and new components. The presence of any 
original components which could not have been formed by the meta- 
morphic process indicates that the forces were not intense enough, 
or did not last long enough, to render the alteration complete; we 
have then an incompletely metamorphosed rock. On the other 
hand, the presence of original quartz grains alongside new quartz (as 
is frequently observed) is no proof of incomplete metamorphism, 
because quartz, which is stable throughout a very wide range of 
conditions, may appear in spite of considerable variation of chemi- 
cal composition, temperature, or pressure. Again, it is a question 
of the stability of the new forms. When one recalls how frequently 
metastable forms appear in laboratory work and considers the 
special circumstances of metamorphism, one need not be surprised 
if metastable substances occur frequently in nature also even though 
the conditions of formation remain undisturbed for long periods of 
time. Metastable forms are observed oftenest with polymorphic 
or hydrated substances; their occurrence therefore in general 
changes only the nature, but not the number of the phases present. 
For instance, so far as the phase rule is concerned, it does not matter 
though kyanite appear instead of the stable sillimanite.’ 
Metamorphic processes are merely the result of a tendency 
toward a new equilibrium position; whether this new position of 
equilibrium is reached or not, depends, however, on a number of 
factors. It is a question if a rock as a whole can always be con- 
tThe simultaneous occurrence of both sillimanite and kyanite would of course 
increase the number of phases by one. 
