SPECIAL POEMS OP STEUCTUEE. 
bOl 
Amygdaloid ,—This is also another form of igneous rock, 
admitting of every variety of composition. 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 derives its name from the Greek word 
arnygdalon,^ an almond. The origin of this structure can not 
be doubted, for we may trace the process of its formation 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, or infiltered by wa¬ 
ter permeating the rock. As these bubbles have been some¬ 
times lengthened by the flow of the lava before it finally 
cooled, the contents of such cavities have the form of al¬ 
monds. In some of the amygdaloidal traps of Scotland, 
‘ where the nodules have decomposed, the empty cells are 
seen to have a glazed or vitreous coating, and in this respect 
exactly resemble scoriaceous lavas, or the slags of furnaces. 
The annexed figure represents a fragment of stone taken 
from the upper part of a sheet 
One-half is scoriaceous, the 
pores being perfectly empty; 
the other part is amygdaloid¬ 
al, the pores or cells being 
mostly filled up with carbonate 
of lime, forming white kernels. 
Lava ,—This term has a 
somewhat vague signification, 
having been applied to all 
melted matter observed to 
flow in streams from volcanic 
vents. When this matter con¬ 
solidates in the open air, the 
upper part is usually scoria¬ 
ceous, and the mass becomes 
more and more stony as we 
descend, or in proportion as it 
has consolidated more slowly and under greater pressure. At 
the bottom, however, of a stream of lava, a small portion of 
scoriaceous rock very frequently occurs, formed by the first 
thin sheet of liquid matter, which often precedes the main 
current, and solidifies under slight pressure. 
The more compact lavas are often porphyritic, but even 
the scoriaceous part sometimes contains imperfect crystals, 
which have been derived from some older rocks, in which 
of basaltic lava in Auvergne. 
Fig. 587. 
Scoriaceous lava in part converted into 
an amygdaloid. Montague de la Veille, 
Department of Puy de Dome, France. 
