CRYSTALLIZATION OF IGNEOUS ROCKS. 
Ill 
The relation of rock structures to the geological occurrence 
of glassy fine-grained and coarse-grained rocks. 
The instances where all of the crystallization took place 
after the magma reached to within a short distance of the 
surface of the earth. 
The relation of rock structures to the chemical composi¬ 
tion of the rock, showing its effect on the extent of crystal¬ 
lization and the character of the structure. 
The relation of the mineral composition of rocks to their 
chemical composition for rocks of like geological occurrence, 
with a notice of certain exceptions. 
The relation of the mineral composition of rocks to their 
geological occurrence, and the consequent variation of the 
mineral composition of rocks having like chemical compo¬ 
sition. 
In discussing the probable causes that have led to the 
crystallization of these rocks attention was first turned to 
the results of direct experimentation, which show— 
That certain basic rocks and their component minerals 
can be produced by the gradual cooling of their fused ma¬ 
terial under slight pressures. 
That the pyroxenes have a great tendency to crystallize 
under such conditions, and that nepheline and lime-soda 
feldspars crystallize less easily. 
That certain minerals characteristic of the acid rocks have 
not yet been produced in this way. 
That the latter minerals have been produced with the aid 
of mineralizing agents. 
From analogy with the crystallization of saturated solu¬ 
tions of salts it seems highly probable that molten rock 
magmas are saturated solutions of silicate compounds and 
certain oxides. 
The exact laws governing the crystallization of salts from 
mixed solutions have not been determined, and nothing is 
known regarding the actual behavior of rock magmas under 
different physical conditions. It is assumed that Sorby’s 
