82 
IDDINGS. 
Relations between the Mineral Crystals, (a.) Association as 
Inclusions and Inter growths. —(1.) The tendency of certain 
minerals to inclose particular substances has already been 
alluded to. It may be well to notice some of the commonest 
of these associations in igneous rocks. It is to be remarked 
in general that the nature of the inclusions in minerals 
changes in different occurrences and with different kinds of 
rocks. A few examples will be sufficient, to show the char¬ 
acter of the associations. 
Glass inclusions are widely distributed in most all kinds 
of the rock-making minerals, but are seldom observed in 
mica and almost never in the iron ores. They often occur 
in apatite and zircon, even when the latter mineral is found 
in coarsely crystalline granite. It shows that these minerals, 
which are among the oldest in igneous rocks, crystallized in 
molten magmas. 
In the greater number of rhyolites studied by the writer 
glass inclusions are more numerous in the porphyritical 
quartzes than in the sanidines, and in the majority of glassy 
andesites glass inclusions occur more abundantly in the pla- 
gioclases than in the ferro-magnesian silicates. 
Fluid inclusions, which are specially characteristic of 
coarsely crystalline rocks, are much more abundant in the 
quartzes than in the feldspars, and occur still less frequently 
in the ferro-magnesian silicates. 
Zircon is very often associated with the micas, and mag¬ 
netite abounds in the ferro-magnesian silicates, but is rarely 
met with in the alkali feldspars and quartzes. There are 
instances, however, where iron oxides in minute particles or 
in scales are abundant in certain lime-soda feldspars. 
Augite microlites form characteristic inclusions in a great 
majority of leucites. 
(2.) In the matter of intergrowths it is observed that certain 
minerals are frequently combined, and others almost never. 
Thus quartz and orthoclase are more frequently intergrown 
