AMERICA THE OLD WORLD. 
9 
these two, and called, in consequence of the 
changes thus brought about, the Mctamorphic 
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 after- 
wards 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 lime- 
stone, of clay, and of sandstone have been previ- 
ously 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 pro- 
duced 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, wher- 
ever the stratified rocks have been penetrated by 
the melted masses from beneath. They have 
been themselves partially melted by the contact, 
surd when they have cooled again, their stratifies 
