212 ALEXANDER N. WINCHELL 
closely to gabbro, peridotite, or basalt, or their alkaline equivalents, 
and may be regarded as variations of such types. 
The second co-ordinate extends from left to right across the 
paper and relates to mode of occurrence and conditions of forma- 
tion; here again three classes are established, namely, plutonic, 
hypabyssal, and volcanic. Plutonic rocks solidified at consider- 
able depth and have been later uncovered and brought to view 
through erosion. They form batholiths, stocks, and laccoliths. 
Hypabyssal rocks crystallized at moderate depth in the form of 
dikes, sills, or intrusions of irregular shape called chonoliths;! they 
are often called dike rocks. Volcanic rocks were brought to the 
surface (or near it) by volcanic action and form surface flows or 
intrusions near the surface. They also form beds and irregular 
aggregates through the accumulation of fragmental materials 
thrown out of volcanoes. 
Plutonic rocks have granitic texture, that is, the essential con- 
stituents are crystalline anhedra of irregular shape and similar sizes, 
or dissimilar sizes showing gradations without a break from the 
smallest to the largest. Hypabyssal rocks may be divided into 
three types: the felsic, the aschistic, and the mafic. The aschistic 
or undifferentiated hypabyssal rocks commonly have porphyritic 
texture with abundant large phenocrysts and coarse granitic or 
aplitic groundmass. The felsic and mafic hypabyssal rocks com- 
monly have aplitic texture, that is, they consist chiefly of fine evenly 
granular euhedral constituents, wholly crystalline. Some of these 
rocks have pegmatitic texture; others have porphyritic texture. 
The volcanic rocks commonly have glassy, felsitic, trachytic, or 
porphyritic textures; certain types have ophitic texture, in which 
the plagioclase feldspars in lath-shaped crystals are partly or wholly 
inclosed by mafic minerals. 
The third co-ordinate exténds from top to bottom of the sheet 
and relates to mineral composition; on this basis igneous rocks are 
divided into three primary groups: first, those in which alkali feld- 
spar is dominant or in which felsic minerals including lenads are 
dominant, second, those in which soda-lime feldspar is equal to or 
dominant over alkali feldspar, or mafic minerals are equal to or 
™R. A. Daly, Jour. Geol., XIII (1905), 498. 
