32 
ELEMENTS OF GEOLOGY. 
der 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 crystallized, but with extreme 
slowness, and under conditions very dilferent 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 en¬ 
tangled gases gives rise in ordinary lava. 
Metaniorphic, 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 scorise, or angular 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 sediment¬ 
ary formations, and are therefore said to be stratified. The 
beds sometimes consist of an alternation of substances vary¬ 
ing in color, composition, and thickness, precisely as we see 
in stratified fossiliferous deposits. According to the Hut- ^ 
tonian theory, which I adopt as the most probable, and 
which will be afterwards more fully explained, the materi¬ 
als 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 com¬ 
plete 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 Avith 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 evident¬ 
ly 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 
