CRYSTALLIZATION OP IGNEOUS ROCKS. 
79 
(2.) The mineral substance of the crystals differs in differ¬ 
ent individuals; sometimes in those of the same species in 
one rock-section. Usually the internal character is the 
same for all the individuals of the same species in one rock. 
The substance of a mineral is seldom free from inclusions of 
other substances. The foreign material may be in a gaseous, 
fluid, or solid condition. Gases exist in minute cavities in 
all the minerals to a greater or less extent. Sometimes they 
appear to be entirely absent. Often they occur so uniformly 
distributed through particular minerals in certain rocks as to 
become characteristic of them. The cavities may be irregu¬ 
larly shaped or have the form of negative crystals. The 
nature of the gases has seldom been investigated chemically. 
Fluids exist in cavities similar to those occupied by gases. 
They usually contain a gas-bubble, the size of which, as com¬ 
pared with the amount of fluid, varies greatly, even in the 
same crystal. The liquid is generally water, which may 
contain a variety of substances in solution, some of which 
crystallize out. Besides water there is often liquid and gas¬ 
eous carbon-dioxide in the same cavity. 
Gaseous and fluid inclusions are occasionally of secondary 
origin and accompany the decomposition of the rock. 
The solid inclusions may be amorphous—that is, glassy— 
or they may be crystallized. 
Glass inclusions are usually found in the minerals of glassy 
rocks or in those whose groundmass is fine-grained. They 
are evidently portions of the magma of the rock inclosed 
during the crystallization of the mineral. They are usually 
rounded or irregularly shaped, but may occupy negative 
crystal cavities. This is especially the case in quartz, where 
they generally have dihexahedral forms. They often con¬ 
tain a gas-bubble, and sometimes carry nine or ten in one 
inclusion. 
Inclusions resembling those of glass in form and manner 
of occurrence, but consisting of crystalline grains, are evi¬ 
dently inclusions of magma, which, instead of solidifying as 
glass, crystallized to an aggregate of minerals like those in 
