THE CLASSIFICATION OF THE IGNEOUS ROCKS iife) 
described types for which Rosenbusch has named a new family * 
are certainly remarkable types, but except for the quantities of 
silica, iron, and lime which they contain, they are as different 
from one another as two rock types can be imagined to be. 
Ijolite is rich in soda and alumina, Missourite poor ; Ijolite is 
poor in potash and magnesia, Missourite rich to excess in both. 
Comparison of their diagrams with those represented in Plate IJ 
shows that they are the end members of the Shonkinite-Essexite 
series, Missourite fitting almost perfectly into the series, being 
only a trifle low in lime, and Ijolite failing to do so only being 
too high in lime and a bit too low in iron. 
The igneous rocks of granitic texture when examined chemically 
fall, therefore, quite naturally into three progressive series, which have 
distinct and common characteristics. —These series may provisionally 
be designated by the limiting families of each, as the granite 
nephelene-syenite, missourite-ijolite, and syenite-gabbro series 
(Plate IV). The peridotites and pyroxenites do not fall per- 
fectly into any of the three, but are yet closely allied to the 
syenite-gabbro series. 
Granite-nephelene-syenite series Missourite-ijolite series Syenite-gabbro series 
Granite family Missourite family Syenite family 
Alkali-syenite se Shonkinite Diorite oy 
Nephelene-syenite “ Theralite Gabbro s 
Essexite of — 
Ijolite if Pyroxenite family 
Peridotite S 
The composite diagrams of the granite-nephelene-syenite 
and syenite-gabbro series are shown in Plate IV, those of the 
Missourite-ijolite series in Plate V. The common characteristics 
of each of the series are well brought out by averaging the com- 
posites of the several members in each to form series composites, 
as has been done in Plate VI. 
Composites of certain igneous rock types having rhyolitic texture.— 
No comprehensive attempt has yet been made to determine simi- 
lar relationships among the rocks of rhyolitic texture, but com- 
posites of a considerable number of the specific rock types of 
* ROSENBUSCH : Elemente der Gesteinslehre, p. 179. 
