PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 485 
topics only in so far as they are important in regard to metamorphic 
processes.* 
Furthermore, the references to previous work are in no wise to 
be understood as a bibliography of the subject; they are given 
merely as illustrations, or expansions, of the ideas in the text, and 
have been chosen from the work with which the writers happen to 
be most familiar. 
In pursuance of this plan, we give first a brief discussion of the 
general effects of temperature and pressure, followed by sections 
dealing with the general behavior of systems when exposed to 
(t) uniform pressure, (2) non-uniform compression or stress. 
Chemical composition is always specific and characteristic of the 
particular system; therefore it cannot be discussed in a general 
way, but must be ascertained by experiment for each system. A 
similar remark applies to rate of reaction. 
EFFECTS OF TEMPERATURE AND PRESSURE ON SYSTEMS 
SOLID-SOLID” 
General considerations.—A very large number of all crystalline 
substances exist in more than one crystalline form;’ thus silica 
appears in at least seven distinct forms, sulphur in at least four 
solid forms, and so on. And it is a matter of common knowledge 
that further and more extended investigation always swells the 
list of polymorphic substances; so that it would appear as if poly- 
t For fuller information on the chemical topics the reader is referred to textbooks 
of theoretical chemistry: the more elementary books of Walker, Introduction to 
Physical Chemistry (Macmillan), or Senter, Outlines of Physical Chemistry, the larger 
general works of Ostwald, Allgemeine Chemie, or Nernst, Theoretische Chemie, or the 
more special books, such as Roozeboom’s Heterogene Gleichgewichte or those in the 
series edited by Ramsay and published by Longmans; on the petrologic side, to books 
such as Harker, Natural History of Igneous Rocks, Elsden, Principles of Chemical 
Geology, and especially to Grubenmann, Die kristallinen Schiefer. Doelter’s new 
Handbuch der Mineralchemie is a collection of observations, rather than a critical 
discussion of the subject, and it is not entirely free from errors of interpretation. 
2 Throughout this paper the term solid is used to designate crystalline material, 
as distinct from amorphous material (glasses); the latter are merely subcooled liquids, 
which differ from ordinary liquids only in that their viscosity is almost immeasurably 
greater. 
3 A fairly complete list of the instances known up to 1$93 will be found in Arzruni, 
Physikalische Chemie der Krystalle (Braunschweig, 1893). 
