286 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
vidual minerals, the rules that in the upper physico-chemical zone the 
alterations upon the average result in expansion of volume and in the 
lower zone the alterations result in the contraction of volume are be- 
lieved to hold and to be of fundamental importance in the metamor- 
phism of rocks. 
This principle of the development of minerals of high specific gravity 
at depth and of low specific gravity near the surface has a direct appli- 
cation to the crystallization of magmas. From a magma ofa given chem- 
ical composition there can be little doubt that the greater the depth, and 
therefore the greater the pressure at which crystallization occurs, the 
higher the average specific gravity of the compounds which form, or, in . 
other words, the greater the specific gravity of the rocks. However, in 
this paper no attempt will be made to work out the applications of the 
rule to individual minerals and rocks. Although other factors enter into 
the phenomena, the presence of glass in rocks which have crystallized 
from magmas at and near the surface, and the absence of glass and 
the presence of more dense, crystallized minerals in the deeper-seated 
rocks, is perhaps the most striking single illustration of the truth of the 
principle.* 
METAMORPHISM FROM THE DYNAMIC POINT OF VIEW 
From the foregoing summary it is evident that there are various lines 
of thought which a discussion might follow. In the treatment of the 
entire subject of metamorphism, from which the present paper is adapted, 
an attempt is made to follow several of them; but in the present paper 
I shall discuss certain phases of the subject of metamorphism from the 
point of view of dynamic action. 
Dynamic action is of two kinds—molecular dynamic action and mass 
dynamic action. By molecular dynamic action is meant interchange 
between the molecules. Metamorphism by such interchange has gener- 
ally been called static metamorphism. Molecular dynamic action is 
always accompanied in some degree by mass dynamic action. By mass 
dynamic action is meant deformation of the body of the rocks. To 
* The above conclusion as to the condensation of material at considerable depths has an impor- 
tant bearing upon Reade’s theory of mountain making. (The Origin of Mountain Ranges, by T. 
Mellard Reade: London, 1886.) His explanation of the rise of mountains is that the volume of 
the thick deposits of sediments increases as a consequence of the rise of the isogeotherms. I 
believe that possible expansion due to this cause is more than compensated in the case of the sedi- 
ments by the mechanical bringing of the particles closer together as the result of pressure, in 
many instances to the practical disappearance of the interspaces, and by the condensation of the 
material itself by the physico-chemical changes above explained. The condensation also has a 
bearing upon estimates of crustal shortening. In so far as condensation occurs, shortening of the 
outer crust of the earth may allow accommodation to a nucleus of decreasing size without crustal 
corrugation. 
Sk, 
ee 
