194 IGNEOUS ROCKS 



occur in small quantities and which may be present or absent, 

 without affecting the character of the rock. The distinction is 

 necessary and useful, but is sometimes arbitrary. 



Another necessary distinction is that between original and 

 secondary minerals. Original minerals were formed with Or be- 

 fore the rock of which they are constituents, and secondary 

 minerals are produced by the alteration or reconstruction of the 

 original ones. 



With comparatively few exceptions, the igneous rocks are made 

 up of some felspar or felspathoid, together with one or more of 

 the pyroxenes, amphiboles, micas, or quartz. Magnetite is also 

 very common. 



What was said above with regard to the difficulty of classifying 

 rocks, applies more especially to the igneous group, because of 

 the way in which the various kinds shade into one another, since 

 even the same molten mass may differentiate into several species, 

 showing not only differences of texture, but marked changes of 

 chemical and mineralogical composition. In an elementary work, 

 like the present, only a meagre outline of the subject can be 

 attempted, for the microscopic study of rocks, or petrography, 

 has now become an independent science of great scope and inter- 

 est, and cannot be compressed into a few pages. 



The classification of the igneous rocks now most generally 

 adopted, is made upon a threefold method, according to texture, 

 and chemical and mineralogical composition. In the following 

 table (modified from Kemp's) the textures are given in vertical 

 order, while transversely the arrangement is mineralogical, chiefly 

 in accordance with the principal felspar. In this manner the 

 acidic rocks come at the left side of the table and the basic at the 

 right side. The percentages of silica are given on a lower line of 

 the table. 



