Classification of Massive Rocks. 215 
of the granites. The porphyrytic constituents are quartz, ortho- 
clase, and usually one or more of the granitic minerals, biotite, 
hornblende, augite or muscovite. The same minerals occur also 
in the ground mass. 
No attempt is made to divide the granite porphyries, although 
it may be convenient to separate them into :— 
1, GRANITE PORPHYRY proper, containing no muscovite, and 
2, ELVAN muscovite—rich varieties. 
B. THE SYENITE PORPHYRIES. 
The syenite porphyries differ from the granite porphyries in 
the absence of quartz from among the porphyritic crystals, In 
all other respects they are similar to them. Quartz occurs in 
the groundmass, and plagioclase is more common than in the 
granite porphyries. 
They are divided, according to their principal iron-bearing 
constituent, into :— : 
1. HORNBLENDE SYENITE PORPHYRY, which contains orthoclase 
and hornblende as the most prominent porphyritic 
constituents, 
2. MICA sYENITE PORPHYRY, in which orthoclase and biotite 
occur in porphyritic crystals. 
3. AUGITE SYENITE PORPHYRY, in which augite is the most 
important non-feldspathic porphyritically developed 
component. 
C. THE ELEOLITE SYENITE PORPHYRIES. 
The elæolite syenite porphyries usually contain eleolite as the 
Most important porphyritic constituent after orthoclase. In one 
or two cases the elæolite is found only in the groundmass. 
Rocks belonging to this family have not been sufficiently studied 
to admit of further classification. 
D. THE DIORITE PORPHYRITES. 
The diorite porphyrites are not very widespread. They consist 
of plagioclase, hornblende, and sometimes quartz and biotite as 
Porphyritic crystals in a groundmass composed principally of 
Plagioclase and quartz. 
They are divided into families, in accordance with the exist- 
ence or non-existence of quartz among the porphyritic crystals, 
