31. _Trachyte_ (_trachys_, rough) is a pale or dark-gray rock, harsh
and rough to the touch, in which felspar is the predominant mineral. It
is a common product of eruption in modern volcanoes.
_Clinkstone_ or _phonolite_ is a greenish-gray, compact, felspathic
rock, somewhat slaty or schistose, and weathers with a white crust. It
gives a clear metallic sound under the hammer. It is a rock not met with
among the older formations of the earth's crust, being confined to
Tertiary (see table, p. 85) or still more recent times.
_Obsidian_ or _volcanic glass_ is usually black, brown, or green, and
usually resembles a coarse bottle-glass. When it becomes vesicular, it
passes gradually into the highly porous rock called _pumice_. It is
eminently a geologically modern volcanic rock.
_Felstone_ is a reddish-gray, bluish, greenish, or yellowish, hard,
compact, flinty-looking rock, composed of potash-felspar and silica. It
is generally splintery under the hammer. Some varieties are slaty, and
are frequently mistaken for clinkstone, which they closely resemble.
When the quartz in felstone is distinctly visible either as grains or
crystals, the rock passes into a _quartz-porphyry_.
_Granite_ is recognised as an igneous as well as a metamorphic rock.
Sometimes the veins and dykes which proceed from or occur near a mass of
granite contain no mica--this kind of rock is called _elvan_ or
_elvanite_.
_Porphyrite_ or _felspathite_ includes a number of rocks which have a
felspathic base, through which felspar crystals are scattered more or
less abundantly. Sometimes hornblende, or augite, or mica is present.
The colour is usually dark--some shade of blue, green, red, puce,
purple, or brown--and the texture varies from compact and finely
crystalline up to coarsely crystalline. Porphyrites are usually
porphyritic, and frequently amygdaloidal.