PYROC RYSTA LLINF. ROCKS. 
49 
denotes the presence of this mineral. Its composition is repre- 
sented below: 
Epidote. 
Silex, 37-0 
Alumina, 21*0 
Lime, 15‘0 
Oxide of iron, 24**0 
Manganese. 1*5 
It is closely allied in composition to pyroxene. 
GROUPS OF PYROCRYSTALLINE ROCKS. 
§ 41. Three kinds of feldspar, the prismatic, albite, and labra- 
dorite, are frequently associated with hornblende and pyroxene. 
They form groups which belong to the later formed rocks, or to 
the pyroplastic rocks. Hornblende, taking the place of mica in 
granite, forms a compound which is called sienite, and it may 
be surmised that it is not the oldest kind of granite. The feld¬ 
spars, when associated with hornblende and labradorite, con¬ 
stitute the greenstones, porphyries, basalt, and trap. These 
associations do not appear to have been formed at all in the 
'earliest consolidations of the earth’s crust. 
§ 42. Feldspar, however, is not confined to the greenstones 
and basalts. We find it incorporated with many of the later 
formed pyroplastic rocks. Thus the claystones and clinkstones 
are compact rocks, in which the elements of feldspar predomi¬ 
nate. The latter, when struck with a hard body, rings like a 
piece of baked earthenware. The first is often a porphyry. 
The rock which is known as petrosilex is a reddish compact 
feldspar, spotted with crystals of white feldspar. Trachyte 
being composed of hornblende and glassy feldspar, belongs to 
this group. Diallage rock might be placed in this connection, 
as it is composed of diallage and feldspar; but geologically, it 
is associated with serpentine, and seems in this respect to be 
separated from the true greenstones. 
The dolerites are regarded by some geologists as combina¬ 
tions of labradorite and augite. They may be placed therefore 
with the pyroxenic compositions, and also as associates of the 
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