Page:Popular Science Monthly Volume 27.djvu/849

Rh between earthquake crises and volcanic crises is also shown by alternations in their activity. Every volcanic eruption is heralded by precursory tremors, whose violence is calmed down as soon as a volcanic outlet is opened for the escape of the vapors. Now, the vapor of water is the recognized cause of volcanic eruptions, and constitutes, in all parts of the earth, the most abundant and most constant emanation from them. It is the agent that throws out from the depths to the surface the lavas, which, despite their high temperature, hold it incorporated in their paste; in the same way that carbonic acid, dissolved in water, forces the liquid impetuously out of a mineral-water or a champagne bottle; and it also shoots quantities of solid matter, stones, lapilli, and cinders, violently into the atmosphere. It is logical to believe that this vapor is likewise the cause of the agitations that accompany volcanic crises. Agreeably to this idea, Kircher and Humboldt regarded volcanoes as safety-valves against earthquakes.

Other countries, again, where there are no volcanoes, are disturbed with no less energy and frequency, and that over great areas. Of such is the southern part of the basin of the Mediterranean. Syria with Palestine, Asia Minor, Turkey in Europe, Greece and the archipelagoes, Italy, Sicily, the southern part of the Iberian Peninsula, and a part of its western coast around Lisbon, have shown evidences of this predisposition within historical times. In each of these countries are districts or places that have been associated with most disastrous convulsions. In them we may discern a common and essential characteristic in the shape of a dislocation of the constituent strata, which is revealed for the most part in a mountainous relief.

In some whole countries the sedimentary strata, which form a notable part of the thickness of the earth's crust, have remained horizontal or nearly so, as they were deposited. In other countries, and over considerable areas, the corresponding strata are raised up, bent, and contorted in different ways, having been subjected to dislocations through enormous thicknesses. Such lifts and foldings can not have taken place in solid masses without being accompanied by many and important fractures. The principal classes of such fractures, which are nearly vertical, are called faults. They crop out and cut the surface of the ground, sometimes for tens and hundreds of miles, and are of indefinite depth, or descend to below where it is possible for man to penetrate; whenever a fault is produced, the two sides are displaced, and must rub hard upon one another; and vast rocky surfaces are thus engraved, striated, and polished: thence they are called mirrors in the language of the miners.

The occurrence of these facts is not confined to mountain-chains, but may be observed in countries that are marked by only slight prominences, but which have undergone similar actions through all their constituent strata. It is evident that the solid envelope of the globe has undergone dislocations at many epochs in its history. The