THE IGNEOUS OR ERUPTIVE ROCKS 65 
depends upon their mode of cooling, and hence their 
position in the earth; (2) according to the minerals 
of which they are composed, these being determined 
in great part by the chemical composition of the lava. 
In the table’ which follows (p. 66), the difference in 
texture is represented by vertical position, the lower 
being coarse grained; and the chemical or mineralogical 
difference is indicated by the horizontal divisions, those 
on the left being most acid, those on the right, basic. 
Names of the minerals contained are given at the 
top of the columns. 
Igneous Rock Structure. — All the igneous rocks, ex- 
cepting volcanic ash and glass, are characterized by a 
erystalline structure, though in some of the fine-grained 
varieties, the crystals cannot be seen without a micro- 
scope. Frequently in the groundmass of glass, or 
almost indistinguishable mineral particles, there are 
larger and quite perfect crystals; such lavas are called 
porphyritic (Fig. 22). These porphyritic crystals often 
haye the perfect outline, with the natural crystal angles 
1This table includes only a few of the most common igneous rocks. To 
be understood they must be handled and studied. Introduction of the less 
common rocks into the table would lead to confusion; for the distinction of 
the various species by the eye is not easy, though with the aid of the micro- 
scope, geologists can detect differences undiscoverable by the natural sight. 
I would suggest a set of rocks to illustrate ash, natural glass, light-colored 
lava (rhyolite or trachyte), dark-colored lava (basalt and diabase), light- 
colored, coarse-grained rock (granite and syenite), dark-colored, coarse- 
grained or plutonic rocks (gabbro and diorite), and porphyritic rock. 
F 
