Granite, already referred to, is another igneous rock, and must have been forced upwards; for as an igneous rock granite has cooled beneath the crust of the earth, throwing out arms, while melting, into other formations, and frequently being found in mountains. There is another kind of igneous rocks formed by the continuous accumulation of the ashes, etc., vomited forth from volcanoes. Masses of mountain are thus produced in the course of years, and the material thus formed is called tufa, or tuff, when consolidated; and this (now solidified) is what caused the destruction of Pompeii and Herculaneum.
Fig. 645.—Eruption of granite.
So we have two classes of Igneous Rocks, the Crystalline and Fragmental (or the “Plutonic” and “Volcanic”), including basalts, pitchstones, pumice, trachytes, granite, syenite, etc.; and, on the other hand, tuff and “volcanic” breccia, with felstones, porphyries, etc., which have been classed as intermediary.
Of these three classes of rocks, the sedimentary and the organic compose the greater portion of the earth. We will now glance at the crust of the earth and its various formations.
Volcanic eruption.