PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 483 
The crystalloblastic structure’ described by F. Becke and by 
U. Grubenmann could not be accounted for if the new-formed rock 
had been formed out of a completely fluid medium, but is easily 
correlated with all the other properties of rocks formed by meta- 
morphism if we regard the change to be the result of a process of 
melting (or of solution) which at any instant is local and partial 
only. 
Metamorphism of rocks is effected by change of temperature, 
of uniform pressure, of stress, and of concentration; it occurs when- 
ever the original components become unstable by reason of the 
changed external conditions, provided always that the velocity of 
reaction is appreciable. The above are the important factors in 
determining any chemical equilibrium, so that in the study of 
metamorphic processes it is necessary only to apply well-known 
physico-chemical principles to these special, and in general com- 
plicated, systems; indeed a beginning in this direction has already 
been made, in the well-known work of Van Hise, Becke, Gruben- 
mann, and V. M. Goldschmidt. 
The opinion has been held that physico-chemical principles are 
incompetent to account completely for the phenomena of meta- 
morphism, for the reason that the system is in many cases not in 
a state of true equilibrium, either during the process of metamor- 
phism or after alterations of the system have practically ceased. 
This lack of stable equilibrium is due to the small rate of reaction 
under the particular conditions, and hence is part of the large 
physico-chemical problem of the relation between rate of reaction 
and the general properties of the system or of its components. 
To grasp the general effects produced by the various agents 
concerned in metamorphic processes is easy, even though quantita- 
tive data are still lacking; but the application of these principles 
to particular cases demands a clear and well-defined knowledge of 
tIt is to be remarked that the words Textur and Strubtur have a signification in 
German which is different from the meaning usually attached to their equivalents 
in English. According to contemporary German usage, which is based on the defini- 
tions given by Becke and Grubenmann, Textur is arrangement in space, while Struktur 
is used to express genetic relationship. Hence crystalloblastic structure has nothing 
to do with textural relationships such as are observed in flow cleavage; for instance, 
the structure of the massive eclogites is typically crystalloblastic. 
