CRYSTALLIZATION OF IGNEOUS ROCKS. 81 
apatite. The plagioclase and hypersthene had crystallized 
at the same time, and therefore under the same conditions; 
yet one was crowded with glass inclusions and the other 
was nearly free from them. 
When two individuals of the same mineral inclose dif¬ 
ferent kinds of substances or very different amounts of the 
same substance it probably indicates that they have crystal¬ 
lized under somewhat different conditions. 
The arrangement of the inclusions in a mineral also throws 
light upon its crystallization. They may be scattered uni¬ 
formly or irregularly through it, or be accumulated in the 
center, or located along planes parallel to crystal faces, or in 
shells corresponding to the form of the crystal in some of 
the early stages of its growth. Alternating zones of inclu¬ 
sions indicate successive periods of rapid and slow crystal¬ 
lization. 
A concentric zonal structure is often noticed in minerals 
free from inclusions. It may be expressed by changes of 
color or by changes of optical properties. It indicates dif¬ 
ferences in the chemical composition of the mineral in suc¬ 
cessive shells of its substance, and is observed in those min¬ 
erals that are known to occur in rocks in isomorphous series, 
particularly the feldspars. 
Such minerals record the vicissitudes of their crystalliza¬ 
tion, when others, like quartz, that do not vary chemically 
and optically, furnish no similar history of their growth. 
Thus a zonally built feldspar may exhibit two or more 
rounded zones that prove that the earlier crystallization 
was checked at some time and the crystal partly resorbed. 
Afterwards the growth continued in crystallographic zones 
until it was again checked. This alternating process may 
go on, and the crystal be finally left with crystallographic 
boundaries. Such a series of events might happen to a 
crystal of quartz without any evidence of it being visible, 
for the chemical composition and optical properties of the 
quartz would be uniform throughout the individual. 
9—Bull. Phil. Soc., Wash., Vol. 11. 
