VOLCANIC ROCKS. 
117 
some hornblende and some iron. It differs from 
basalt in containing a larger proportion of feldspar 
than augite, while the reverse is the case with the 
latter. 
Porphyry is a form of rock very characteristic of 
the volcanic formations. As we have already de- 
scribed it in a former chapter, we need only add, 
that any rock possessing an earthy or compact 
base, through which distinct crystals of one or more 
minerals are scattered, is termed a porphyry. Thus 
trachyte is a porphyry, because it contains distinct 
crystals of feldspar. If the base be greenstone, ba- 
salt, or pitchstone, the rock may be denominated 
greenstone-porphyry, pitchstone-porphyry, &c. 
Amygdaloid, — This compound rock admits of ev- 
ery variety of composition. Indeed, it comprehends 
any rock in which round or almond-shaped nodules 
of some mineral, such as agate^ chalcedony, calcareous 
spar, or zeolite, are scattered through a base of wacke, 
basalt, greenstone, or other kind of trap. It is de- 
rived from the Greek word " amygdala,'^'' an almond. 
We can trace the origin of this structure in modern 
lavas. Small pores or cells are caused by bubbles 
of steam and gas confined in the melted matter. 
After or during consolidation, these empty spaces 
are gradually filled up by matter separating from the 
mass, infiltered by water permeating the rock. The 
almond shape of the globules is owing to the bub- 
bles having been sometimes lengthened by the flow 
of the lava before it finally cooled. Sometimes 
these pores are empty, as in pumice-stone, giving it 
a light, spongy appearance, and a capacity of float- 
ing on water. 
Lava, — This is an indefinite term, having been ap- 
plied to all melted matter which flows in streams 
from volcanic vents. We usually find the upper 
part porous or scoriaceous, from its consolidating in 
the open air ; but, as we descend, the mass becomes 
more solid and stony, owing to the exclusion of 
