ce
of the changes thus brought about, the Metamorphic rocks. The effect of
heat upon clay is to bake it into slate; limestone under the influence
of heat becomes quick-lime, or if subjected afterwards to the action of
water, it is changed to mortar; sand under the same agency is changed to
a coarse kind of glass. Suppose, then, that a volcanic eruption takes
place in a region of the earth's surface where successive layers of
limestone, of clay, and of sandstone have been previously deposited
by the action of water. If such an eruption has force enough to break
through these beds, the hot, melted masses will pour out through the
rent, flow over its edges, and fill all the lesser cracks and fissures
produced by such a disturbance. What will be the effect upon the
stratified rocks? Wherever these liquid masses, melted by a heat more
intense than can be produced by any artificial means, have flowed over
them or cooled in immediate contact with them, the clays will be changed
to slate, the limestone will have assumed a character more like marble,
while the sandstones will be vitrified. This is exactly what has been
found to be the case, wherever the stratified rocks have been penetrated
by the melted masses from beneath. They have been themselves partially
melted by the contact, and when they have cooled again, their
stratification, though still perceptible, has been partly obliterated,
and their substance changed. Such effects may often be traced in dikes,
which are only the cracks in rocks filled by materials poured into them
at some period of eruption when the melted masses within the earth were
thrown out and flowed like water into any inequality or depression of
the surface around. The walls that inclose such a dike are often found
to be completely altered by contact with its burning contents, and to
have assumed a character quite different from the rocks of which they
make a part; while the mass itself which fills the fissure shows by the
character of its crystallization that it has cooled more quickly on the
outside, where it meets the walls, than at the centre.

The first two great classes of rocks, the unstratified and stratified
rocks, represent different epochs in the world's physical history: the
former mark its revolutions, while the latter chronicle its periods of
rest. All mountains and mountain-chains have been upheaved by great
convulsions of the globe, which rent asunder the surface of the earth,
destroyed