Classification of Massive Rocks. 211 
fined to the outer edges of large masses and dyke forms of the kroc. 
The various occurrences of elæolite syenite are not well enough 
characterized to admit of a further classification. 
D. THE DIORITES. 
The diorites may be defined as rocks composed of plagioclase 
and biotite or hornblende, with or without quartz. Orthoclase and 
microcline sometimes accompany the plagioclase, and in certain 
cases augite partly replaces the biotite or hornblende. The struc- 
ture of the diorites departs somewhat from the typical structure of 
the intrusive rocks in that the plagioclase and the biotite, amphi- 
bole and augite are sometimes idiomorphically developed. 
The diorites are divided into :— 
1. Mica Drortres, in which biotite predominates over hornblende, 
including 
(A) mica diorite, which is quartz-free. 
(B) quartz, mica diorite, which is quartz-bearing. 
2. DIORITES, in which hornblende is the most prominent colored 
constituent, including 
(A) diorite, and 
(B) quartz diorite. 
3. AuerrEe DIoRITES, containing a large amount of augite, 
including 
(A) augite diorite, and 
(B) quartz-augite diorite. 
E. THE GABBROS. 
The gabbros are combinations of plagioclase and a monoclinic or 
an orthorhombic pyroxene, with or without olivine. 
Their structure varies slightly from the typical granular struc- 
ture, in that the plagioclase occurs in broad lath-shaped crystals. 
They are divided, according to the nature of their pyroxenic 
constituents into :— 
ABBROS, which contain, as their pyroxenic constituent, dial- 
lage, or a monoclinic augite with a composition 
approaching that of diallage. 
The gabbros include 
(A) gabbro proper, which is olivine: including two 
varieties : 
