pieces of imbedded stone, and no traces of organic bodies, and they are often as
crystalline as granite, yet are divided into beds, corresponding in form and
arrangement to those of sedimentary formations, and are therefore said to be
stratified. The beds sometimes consist of an alternation of substances varying
in colour, composition, and thickness, precisely as we see in stratified
fossiliferous deposits. According to the Huttonian theory, which I adopt as the
most probable, and which will be afterwards more fully explained, the materials
of these strata were originally deposited from water in the usual form of
sediment, but they were subsequently so altered by subterranean heat, as to
assume a new texture. It is demonstrable, in some cases at least, that such a
complete conversion has actually taken place, fossiliferous strata having
exchanged an earthy for a highly crystalline texture for a distance of a quarter
of a mile from their contact with granite. In some cases, dark limestones,
replete with shells and corals, have been turned into white statuary marble; and
hard clays, containing vegetable or other remains, into slates called mica-
schist or hornblende-schist, every vestige of the organic bodies having been
obliterated.

Although we are in a great degree ignorant of the precise nature of the
influence exerted in these cases, yet it evidently bears some analogy to that
which volcanic heat and gases are known to produce; and the action may be
conveniently called Plutonic, because it appears to have been developed in those
regions where Plutonic rocks are generated, and under similar circumstances of
pressure and depth in the earth. Intensely heated water or steam permeating
stratified masses under great pressure have no doubt played their part in
producing the crystalline texture and other changes, and it is clear that the
transforming influence has often pervaded entire mountain masses of strata.

In accordance with the hypothesis above alluded to, I proposed in the first
edition of the Principles of Geology (1833) the term "Metamorphic" for the