PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 623 
of the ejected blocks found in the Laachersee region. Such cor- 
rosion phenomena are also common, for instance, in biotites of 
injected rocks. This action of hot gases is identical with that of 
a blast flame, except that some addition of material may at the 
same time take place. 
CONCLUSION 
In the foregoing pages we have endeavored to set forth the most 
important general principles concerned in rock metamorphism— 
a general term which includes a number of special cases (e.g., 
contact metamorphism) all of which, however, differ only in the 
degree of predominance of one (or more) of a definitely limited 
group of effective factors. These factors are: temperature, 
uniform pressure, stress (non-uniform pressure), and gross composi- 
tion of the system at the time of metamorphism; the same, namely, 
which determine the equilibrium of the relatively simple chemical 
systems hitherto investigated experimentally. The knowledge 
gained from a study of these simple systems may be used as a 
basis for a prediction of the general character and significance of 
metamorphic processes; though in applying the principles one must 
always bear in mind those circumstances which oppose the attain- 
ment of a state of true equilibrium, such, for example, as slowness 
of reaction or the formation of metastable intermediate products. 
Now, although the general character of the process may be 
predicted, no particular statement as to the effects produced in a 
given system by change of any of the above factors can yet be 
made, owing to lack of the requisite quantitative data. In this 
connection it is to be noted that the general application of experi- 
mental results which obtain for a given system under given external 
conditions to another-system under similar conditions, or even to 
the same system under widely differing conditions, is subject to 
considerable limitation. Conclusions drawn from such extra- 
polation of experimental evidence will commonly be of little value, 
and may be altogether misleading; moreover, one may as well 
guess the final result as arbitrarily choose the data required in 
‘calculating it. From this we see that the application of the above 
simple principles, which determine rock metamorphism, to the 
complicated rock systems will be no simple matter, but will require 
