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.
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