616 JOHN JOHNSTON AND PAUL NIGGLI 
The term metamorphism with addition of material we reserve 
for the changes characterized by a penetration of the original rock 
by some foreign system; we do not use it to denote the small local 
changes in concentration which may have taken place. The 
foreign system, the viscosity of which must be comparatively small, 
will generally be of magmatic origin; we must therefore briefly 
consider magmatic processes. 
Magmas are mixed solutions of silicates, charged with volatile 
substances, the so-called gas mineralizers, the amount of which, 
varying with the progress of the intrusion and solidification, 
depends upon the previous history and original composition of the 
particular magma basin. Of these volatile components, the major- 
ity can exist, when alone, only as gases at the temperature at which 
the magma crystallizes. 
Analysis of volcanic exhalations has led to the recognition of the 
following gases and vapors, excluding those of minor importance: 
HO, H., N;, CO, CO; His; SO; HCl, CL, CH, HIF Sik. \V/nick 
of these appear under given conditions depends mainly upon 
temperature and pressure, the latter especially, since it influences 
the equilibrium greatly when the reaction is accompanied by a 
change in the total number of molecules (especially therefore in 
cases of dissociation or association). Conversely, reaction will 
frequently take place in gaseous systems when the pressure changes, 
therefore in magmatic gases when they escape into a region of 
lower pressure. Such reactions are often attended by a large 
evolution of heat, which is doubtless one of the sources of heat, 
and possibly even one of the causes of volcanic outbreaks. For 
any reaction which evolves much heat and proceeds very rapidly 
may be explosive; for instance, hydrogen and oxygen explode 
when set off by a spark, that is, when the reaction is started at a 
high temperature (where its rate is very great), the amount of 
heat evolved by the reaction being sufficient to prevent a lowering 
of the temperature of the zone reacting at any moment. When 
one takes into account the reactivity of gases and the dependence 
of the position of equilibrium at any moment upon the effective 
temperature and pressure at that time, one is led irresistibly to 
the view that from the gases found in the exhalations no safe con- 
