As a corollary to these, the hypothesis of the upheaval of the earth’s
crust follows--upheavals having produced local revolutions. The result
of these upheavals has been to superimpose new materials upon the older
rocks, introducing extraneous rocks called _Eruptive_, beneath, upon,
and amongst preceding deposits, in such a manner as to change their
nature in divers ways. Whence is derived a third class of rocks called
_Metamorphic_ or altered _rocks_, our knowledge of which is of
comparatively recent date.