THE CLASSIFICATION OF THE IGNEOUS ROCKS Is 
STUDY OF THE COMPOSITES OF THE PRINCIPAL FAMILY TYPES OF 
THE IGNEOUS ROCKS HAVING GRANITIC TEXTURE 
The composite diagram may be made to represent either 
specific or family types according to the analyses which are 
combined to produce it. By combining separately analyses of 
the principal species of granite, viz., alkali-granite, muscovite- 
biotite-granite, biotite-granite, hornblende-granite, and augite- 
granite, we are prepared to draw the composite diagram of each 
and can then compare them with one another ; or, if we choose, 
we may compose all to form a single composite, which then rep- 
resents not a specific but a family type—granite. These granite 
composites may be studied in Plate I. The alkali-granite com- 
posite is composed from six analyses, the muscovite-biotite- 
granite from two, the biotite, hornblende, and augite-granites 
each from four, so that the family composite is made from the 
average of twenty analyses. 
The composites of each of the families of the igneous rocks 
having granitic textures may be similarly prepared and studied 
in connection with one another. (See Plate II). The family 
types Selected, viz., granite, syenite, alkali-syenite, nephelene- 
syenite, shonkinite, theralite, essexite, diorite, gabbro (including 
hypersthene-gabbro and norite), pyroxenite, and peridotite, 
when seen in their composites allow their peculiar characteris- 
tics to be read at a glance. 
The granites are distinguished from all the other families by 
their excess of silica and, moreover, by the small quantities of the 
protoxide bases and moderate amounts of alumina and the alka- 
lies. The grantes, alkali-syenites, and nephelene-syenites form a 
progressive series which ts characterized by decreasing silica and 
rapidly increasing soda and alumina, and to a less degree by wcreas- 
ing potash and lime, so that the alkali and nephelene-syenite rocks 
become preéminently the alkalt-alumina rocks. 
The shonkonites, theralites, and essexites form a second progressive 
series in which the silica and tron remain nearly constant but in which 
the potash, magnesia, and lime steadily decrease as the soda and 
alumina increase. The essexites are essentially alkali-diorites 
