62 
AMERICAN GE0L0GV. 
part to one kind of structure, and in part to another; still the 
examples described in the foregoing paragraphs, constitute the 
distinctive kinds of structural arrangement, which result from 
the common action of heat upon rocks, and the structure which 
results from the different degrees of it, furnish the grounds upon 
which they may be separated into classes or groups. 
We have already taken occasion to speak of structures, as 
affording indications of the age of the pyrogenic rocks, and it 
appears that at one extreme of time the rocks formed were all 
crystallized, while at the other extreme they all want it. The 
first belongs to the most remote period which any of the geolo- 
gical phenomena recorded in earth’s history, have furnished; 
the last belongs to the present, or to the action of the present 
periods, and form only lavas, slags, sands, and porous products, 
but no granites. The pyrocrystalline, therefore, differ from 
other rocks of the pyrogenic kind, in structure and age, though 
they are not confined to one age or period. By the modification 
of structural arrangements under the influence of heat, varying 
in intensity, we may separate the pyrocrystalline rocks into two 
groups. In the first the massive structure prevails, in the second 
the laminated. These distinctions have been already illustrated. 
The special characters of each group, together with its mem¬ 
bers, will be given in the proper place. 
CHARACTERISTICS OF THE PYROGENIC OR ERUPTIVE ROCKS. 
§ 49. The massive 'pyrocrystalline rocks . The first charac¬ 
teristic possessed by this class of rocks is, the perfect separation 
of each individual of the mass, by crystallization. The second 
is the indiscriminate arrangement or mixture of the minerals, 
without regard to lamina, bands or stripes. They are composed 
of feldspar, mica, quartz, limestone, hornblende and augite. 
But a separation of parts of the mass is effected by crystalliza¬ 
tion, which has affected the rock as a whole. They are repre¬ 
sented by those which are referred to under the first phase, 
§ 48." 
