Although the old-fashioned ideas as to the association of different minerals held by Breithaupt and others have been found to require much modification, there are still certain rules which hold with a high degree of generality. They are, indeed, merely consequences of the principle that the most important of the factors which determine the mineralogical constitution of an igneous rock, is the chemical composition of the magma from which it is formed. Roughly speaking, we may say that original free silica and acid silicates occur characteristically in acid rocks, more basic silicates in basic rocks.