t 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 sandstone 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 enclosing 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 the animals and plants living u