So much for the Aqueous or Stratified Rocks. Geology next brings before us another and a very different group, of which the origin is ascribed to fire, and which are consequently designated by the title of Igneous Rocks. In their general appearance they are chiefly distinguished from the former by the absence of regular stratification; but they are, nevertheless, intersected by numerous planes of division, or joints, as they are called, and thus divided into blocks of various size and form. Geologists believe that these rocks were at one time reduced to a molten state by the action of intense heat, and afterward allowed slowly to cool and to crystallize. They are divided into two classes, the Plutonic and the Volcanic. The Plutonic Rocks are chiefly granite of some kind or another; and though they now often appear at the surface, they are supposed to have been produced originally at a considerable depth within the crust of the Earth, “or sometimes, perhaps, under a certain weight of incumbent ocean.”[14] The Volcanic Rocks have been formed at or near the surface of the Earth, and, as the name implies, they are usually ejected, in a state of fusion, from the fissures of an active volcano; though not unfrequently they assume the more imposing form of basaltic columns, as at the Giant’s Causeway in Ireland, or on the island of Staffa near the coast of Argyleshire in Scotland.
One group of rocks yet remains to be noticed. They have been called by various names at different times, but are now generally designated by the term Metamorphic. In some respects they resemble the Aqueous Rocks, while, in others, they are more nearly allied to the Igneous. Like the former, they are stratified in their outward arrangement; like the latter, they are more or less crystalline in their internal texture. As to their origin, we are told that they were first deposited under water, like the Aqueous Rocks, and that afterward their internal structure was altered by the agency of subterranean heat. Hence the name Metamorphic, first suggested by Sir Charles Lyell, which conveys the idea that these rocks have undergone a _change of form_. To this group belong many varieties of slate, and also the far-famed statuary marble of Italy.