Nature must and will exercise her sterner as well as her milder powers. In achieving gigantic works, she employs gigantic powers. Her forces are her own; and when she directs them to execute her mandates, she is promptly obeyed. She models and remodels the earth's exterior and interior at pleasure, but never without a beneficent design. Earthquakes break up the earth's crust. Internal fires melt it. Exploding gases lift it. Gravitation moulds it. The atmosphere cools it. The sun and the rain clothe it with verdure; and flowers crown it with beauty. In this way the earth's surface seems to have been prepared for the advent of man, and its interior supplied with coal-fields and reservoirs of oil and gas for his use. Though Nature has made for man ample provision, she requires him not only to help himself, but to take care of himself. Nor does she give him formal notice to keep out of harm's way when she wishes to break up the earth's crust and re-cast it, but proceeds at once. She may sink or elevate a continent at a blow, or she may do it by slow degrees. The earliest writers give us accounts of terrific earthquakes. Thucydides alludes to volcanic eruptions which occurred five hundred years before the Christian era. In the vicinity of volcanic mountains, it has happened that city after city, in the course of ages, has been engulfed, one upon another, in molten lava, or cinders, leaving no record behind them of their unhappy fate. Herculaneum lies buried a hundred feet deep beneath the modern city of Portici; and beneath Herculaneum, a city still more ancient has been discovered, whose name and history are entirely unknown. How many other cities lie buried at the foot of the old fire-crowned monarch of Italy, no one can tell; but doubtless there are several of them. What induced people to occupy a locality so perilous, it is difficult to say, unless it was the superior fertility of a volcanic soil.