80 
IDDINGS. 
the groundmass of the rock. They are more frequent in 
coarsely crystalline rocks than glass inclusions, and do not 
occur in fresh glassy rocks. They are genetically the same 
as glass inclusions, but have passed through different pro¬ 
cesses of consolidation. 
Small crystals or grains of the associated minerals may 
occur as inclusions in any mineral of younger growth. 
The manner of occurrence of inclusions in a mineral throws 
considerable light upon the history of its crystallization. 
Abundant inclusions indicate the rapid growth of a crystal, 
and their absence its slower growth; but this cannot be con¬ 
sidered an infallible criterion, for it is evident that some 
kinds of minerals have a greater tendency to inclose foreign 
substances than others, and that minerals which crystallize 
synchronously, and therefore under the same conditions, take 
up different substances in different amounts. Thus the kind 
of inclusions found in different minerals in the same rock 
may not always be used as evidence that different conditions 
attended the crystallization of these minerals. 
For example, it is often observed that in the same rock 
the quartzes carry numerous glass inclusions and the feld¬ 
spars few, if any, and more probably gas inclusions. In these 
cases it would not be correct to assume that the quartzes 
crystallized when the magma was in a different condition 
from that in which the feldspars formed; for it has been 
observed that where quartz and orthoclase crystallized to¬ 
gether synchronously and formed a pegmatoid porphyritical 
crystal, as in one instance in Eureka, Nev., the quartz sub¬ 
stance inclosed glass in dihexahedral cavities, and the feld¬ 
spar substance inclosed gas in irregularly shaped cavities. 
Another instance is that of intergrown plagioclase and 
hypersthene in a glassy andesite from Mt. Hood, Oregon. 
The plagioclase, in common with other minerals in the rock, 
carried abundant sharply defined glass inclusions, with some 
apatite and an occasional grain of magnetite; while the 
hypersthene carried very few glass inclusions, which were 
poorly defined, but numerous grains of magnetite and some 
