Page:Popular Science Monthly Volume 27.djvu/850

828 manifestations of gigantic force thus plainly registered are the effects of ancient back-foldings and of lateral or horizontal pressures. It is as if the crust of the earth had become too large for the supporting nucleus, and had, to keep in contact with it, to shrink up and bend upon itself. These foldings and fractures have given origin to chains of mountains.

Now, the geological study of earthquakes has shown that their centers of impulsion are in relation with the ground-lines of fracture and dislocation. The disturbed bands are usually longitudinally parallel to the chains. A recent example of this linear disposition has been added in the latest earthquake in Andalusia, the major axis of which, according to M. Fouqué, is parallel to the mountain-crests of the province, as well as to the numerous faults that cut it up. Another important point to be noticed is that the countries in which the mountains have most recently acquired their latest relief are the ones in which these subterranean agitations are particularly frequent.

The Andalusian masses which have been so rudely disturbed within a few months partake of all the structural conditions that have just been noticed. The Sierra Nevada is among the youngest chains of mountains on the globe. The tertiary strata around it have been powerfully lifted up, sometimes to more than three thousand feet above the sea, without having their horizontality destroyed. According to M. de Botella there are also, in different places at the foot of the chain, strata regarded as quaternary that have been tilted into an inclination of 65°. Furthermore, numerous faults furrow the country, while the parts that have been most disturbed, according to Macpherson, are upon the faults that terminate the crystalline mass of the Sierra Tejea and Almijara. The numerous thermal springs of the region are further evidences of the deep fractures that traverse it.

Similar conditions, dislocations, and recent age, are found in other regions subject to subterranean perturbations. They appear notably in that part of the Mediterranean basin which we have spoken of as especially agitated, although it is distant from volcanoes; in the Apennines, the Lebanon, and the mountainous masses of Dalmatia and Croatia bordering on the Adriatic. The configuration of the northern coasts of that sea, so exceptionally slashed and cut by deep indentations, results from the complexity of the fractures that have determined the outlines of their principal features. Even the chain of the Alps, where shocks are felt nearly every year, acquired its final relief only at a comparatively recent epoch. It is conceivable under such conditions that the interior masses are not yet at equilibrium nor wholly subsided, and that they contain vacant spaces affording room for further sinkings.

According to what seems to be the dominant opinion of the day, there are two kinds of earthquakes: those which are due to volcanic actions, and of which the vapor of water is the prime mover; and those which are the effect of such ruptures of the equilibrium of the solid