CRYSTALLIZATION OF IGNEOUS ROCKS. 
75 
spars, and the allied species lencite and nepheline; the 
micas, amphiboles, and pyroxenes; olivine, and the iron 
oxides. With these are associated small amounts of apatite, 
zircon, sphene, the spinels, and other minerals less regularly. 
The great majority of igneous rocks are made up of three 
or more of the essential minerals just named, the varying 
proportions of which give rise to the different kinds of rocks, 
which are generally classified according to their mineral 
composition. 
(2.) The essential minerals, with the exception of the iron 
oxides, are silicates of a small number of chemical elements, 
besides free silica. The chemical composition of the rocks 
consists mainly of silica, alumina, iron oxides, lime, mag¬ 
nesia, soda, and potash, with small amounts of titanic and 
phosphoric acids and traces of a few other compounds. The 
relative proportions of these compounds vary within com¬ 
paratively narrow limits. 
(3.) The igneous rocks may be arranged according to their 
chemical composition in series that rauge from those rich in 
bases and poor in silica to others rich in silica and poor in 
bases. The mineral composition of the rocks necessarily 
corresponds to the chemical composition. Thus basic rocks 
are mostly composed of minerals low in silica, with more or 
less iron oxide and little or no free silica or quartz. The 
acid rocks are largely made up of minerals higher in silica, 
with much quartz and little or no iron oxide. 
But since the mineral constituents are largely composed of 
the same chemical elements in different combinations, it is 
possible to have a variety of different mineral combinations 
that would possess the same chemical composition as a 
whole—that is to say, it is possible to have a number of 
different sets of multiples of the same factors that would sum 
up alike. Hence there is no fixed proportion of certain min¬ 
erals to correspond to each phase of chemical composition of 
the rock. Variations in mineral composition, therefore, occur 
frequently. 
