by some the CRYSTALLINE SCHISTS, in which group are included gneiss, micaceous-
schist (or mica-slate), hornblende-schist, statuary marble, the finer kinds of
roofing slate, and other rocks afterwards to be described.

As it is admitted that nothing strictly analogous to these crystalline
productions can now be seen in the progress of formation on the earth's surface,
it will naturally be asked, on what data we can find a place for them in a
system of classification founded on the origin of rocks. I can not, in reply to
this question, pretend to give the student, in a few words, an intelligible
account of the long chain of facts and reasonings from which geologists have
been led to infer the nature of the rocks in question. The result, however, may
be briefly stated. All the various kinds of granites which constitute the
Plutonic family are supposed to be of igneous or aqueo-igneous origin, and to
have been formed under great pressure, at a considerable depth in the earth, or
sometimes, perhaps, under a certain weight of incumbent ocean. Like the lava of
volcanoes, they have been melted, and afterwards cooled and crystallised, but
with extreme slowness, and under conditions very different from those of bodies
cooling in the open air. Hence they differ from the volcanic rocks, not only by
their more crystalline texture, but also by the absence of tuffs and breccias,
which are the products of eruptions at the earth's surface, or beneath seas of
inconsiderable depth. They differ also by the absence of pores or cellular
cavities, to which the expansion of the entangled gases gives rise in ordinary
lava.

METAMORPHIC, OR STRATIFIED CRYSTALLINE ROCKS.

The fourth and last great division of rocks are the crystalline strata and
slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist,
marble, and the like, the origin of which is more doubtful than that of the
other three classes. They contain no pebbles, or sand, or scoriae, or angular