PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 619 
Under magmatic conditions the volatile substances (such as 
HO, SO., CO,, etc., all of which are to be treated in exactly the 
same way) may in part unite with some of the other constituents to 
form more or less stable compounds which may be liquid under the 
conditions of temperature and pressure obtaining. The proportion 
combined in this way is controlled by temperature and pressure, 
the effect of the latter being to raise the concentration in the gas 
phase and hence, in accordance with Henry’s law, in the liquid 
phase also. An idea of the influence of temperature is afforded 
by the following recently obtained results: in a melt of 1 mol. 
KO and 0.94 mol. SiO, heated in an atmosphere of CO, at 1 atm. 
pressure there is at equilibrium at go00,° 0.33 mol. CO, present as 
carbonate, while at 960° and 1ooo° the corresponding amounts are 
o. 28 and o. 24 mol.’ 
In the magma, therefore, the proportion of gas and liquid is a 
function of chemical composition, of temperature, and especially 
(because of its relation to concentration) of pressure. In the pro- 
cess of crystallization the concentration of that portion of the vola- 
tile components which does not crystallize out as compound or as 
solid solution will increase relatively in the residue. Eventually, 
therefore, a fluid solution under high pressure will remain, which will 
escape whenever an opportunity presents itself, as for instance 
along tectonic lines. When the volatile components can escape 
slowly but continuously they will obviously not be under very 
high pressure, and the amount of material carried by them will be 
small. Consequently in such cases the main effects are due to the 
change of temperature and the production of a more or less homo- 
geneous medium by impregnation with the volatile components 
(especially when water is a factor), which results finally in a fairly 
uniform recrystallization of the original rock. 
The extent of metamorphism is governed in large degree by 
the nature of the original rock; for instance, by its permeability, 
which is doubtless one of the chief reasons why limestones in contact 
with eruptive rocks are much more extensively metamorphosed, 
and contain a greater percentage of pneumatolytic minerals, than 
the shales. Moreover, its chemical composition may be a factor, 
if it is such that reaction may take place between any of its com- 
'P. Niggli, Z. anorg. Chemie (in course of publication). 
