182 VICTOR ZIEGLER 
each other which subsequently enter into the formation of the 
different minerals in the rock, but there also enter in a series of 
volatile components such as water, boron, fluorine, chlorine, etc.— 
the mineralizers. How are these held in the magma? In all 
probability they are in certain molecular combination with some 
of the other oxides present. Under the conditions of great pressure 
and in the presence of mineralizers, a granite begins and completes 
crystallization. The effect of these volatile components is con- 
sidered so important that the solidification temperatures of certain 
granites is placed as low as 2007-350 C.*_ These mineralizers are the 
powerful solvents, they represent to a large extent the “mother 
liquor”? from which the granite crystallized. 
Turn now to the case of a rhyolite. The variable factors are 
quite different. Pressure is far less, the rate of cooling is far more 
rapid. What will be the effect of such a change? In physical 
chemistry we recognize what is known as van’t Hoff’s law, which 
deals with the fact that displacements of equilibrium within a 
solution are effected by changes in temperature and pressure. 
Crystallization in a magma can be brought about in one case at a 
higher and in another case at a lower temperature, depending on 
the pressure. An increase of pressure will induce crystallization 
at a higher temperature. By a change of pressure equilibrium will 
be destroyed within the magma, and reactions taking place will go 
farther in one direction, thus altering the relative proportions of 
the various compounds present. As a result we may obtain in the 
rock an association of minerals quite different from what it would 
have been under a different pressure? As a further result of 
decrease of pressure, the volatile constituents would escape, causing 
additional disturbance in the equilibrium and rearrangement in the 
molecular grouping, which will further influence the final crys- 
tallization product. Thus the law of mass action and van’t Hofi’s 
law seem to prove that granite and rhyolite cannot safely be 
regarded as having the same order of crystallization nor the same 
mineral make-up. The conception of mineralizers strengthens 
this view. 
t Harker, The Natural History of Igneous Rocks, p. 180. 
2 Cf. C. N. Fenner, American Journal of Science, XXIX (March, 1910), 217. 
