208 
IDDINGS. 
It is to be remembered that these irregular variations take 
place not only between allied varieties of rocks, but in differ¬ 
ent parts of the same rock-body, and that they correspond 
to variations in the relative proportions of the essential 
minerals. When it is remembered that the essential min¬ 
erals concerned in this group of rocks are: feldspars, 
pyroxenes, amphiboles, mica, quartz and magnetite, one or 
more of which may be absent from a particular form of the 
rock; and when we consider the number of different com¬ 
plex molecules into which any one of these oxide molecules 
enters, we begin to realize the interdependence of the varia¬ 
tions among the oxide molecules. Thus while most of the 
alumina enters into the composition of the feldspar, a por¬ 
tion of it enters into the ferromagnesian silicates. The 
alkalies are mostly found in the feldspars, but the soda 
takes part in the augites and hornblendes, and the potash 
in the biotite. The lime is an important factor in both the 
feldspars and ferromagnesian silicates; it is most abundant 
in the basic plagioclases, and diminishes as the feldspars 
become more alkaline; it abounds in augite and to a less 
extent in hornblende, and is almost absent from hyper- 
sthene and biotite. The iron and magnesian molecules, 
however, have no part in the composition of the feldspars, 
and are [confined to the ferromagnesian minerals. 
Corresponding to the extremes of the series of chemical 
constituents are the simple minerals, magnetite and quartz, 
consisting of an uncombined oxide or oxides, which act as 
compensators to regulate the exhaustion of the oxide 
molecules in the magma; the magnetite expressing the iron 
oxide in excess of that entering into the complex silicate 
molecules; the quartz representing the excess of silica 
molecules. 
From these considerations we begin to understand the 
relative variations expressed by the diagram. The inverse 
relation between the magnesia and alumina corresponds to 
variations in the complex molecules of feldspars and of the 
ferromagnesian silicates; an increase of feldspar molecules 
is accompanied by a decrease of ferromagnesian molecules. 
