Classification of Massive Rocks. 303 
position of andesite, containing a few microlites cor- 
responding to the porphyritic components of the 
andesite group. 
F. THE BASALTS. 
The basalts are composed essentially of a plagioclase feldspar and 
augite, with or without olivine. They differ from the andesites in 
the predominance of augite over biotite and hornblende, and are 
thus the equivalents of the diabases. 
They possess a wide range of structure, from the hypidiomor- 
phic-granular to the glassy. 
They are separated into two great divisions, distinguished by 
the presence or absence of olivine. 
1, OLIVINE-FREE BASALTS, containing plagioclase and augite in a 
hypocrystalline ground mass. - 
2. OLIVINE BASALTS, in which olivine is an essential constituent, 
in addition to plagioclase and augite. Among the 
olivine basalts are included :— 
(a) hypersthene basalt, with an orthorhombic augite as one 
of the porphyritic constituents. 
(B) quartz basalt, with corroded quartzes among the augite, 
olivine and plagioclase of the first generation. 
(c) hornblende basalt, with brown hornblende as a porphy- 
ritic ingredient. 
4. HYALOBASALTS, or BASALT GLASSES, glasses with the compo- 
sition of basalt, containing microlites of augite, pla- 
gioclase and olivine. 
G. THE TEPHRITES AND BASANITES. 
The tephrites and basanites contain as essential constituents a 
basie plagioclase and nepheline or leucite, or both. The former 
are olivine-free, the latter olivine-bearing. 
They differ from the phonolites in the nature of their prevalent 
feldspar. 
Their most common structure is the holocrystalline porphyritic 
1. THE TEPHRITES are the olivine-free varieties. They are subdi- 
vided, like the phonolites, into :— 
(A) nepheline-tephrite, in which nepheline occurs generally _ 
as a constituent of the ground mass, but occasionally — 
also in porphyritic crystals. 
