Page:Popular Science Monthly Volume 86.djvu/451

Rh The greatest of the Italian earthquakes have been the result of earth fracture accompanied by a succession of destructive shocks. They have been due to the fact that Italy lies within an area characterized by a state of unstable equilibrium. Its mountains and the most lofty mountains of the world, the Himalayas, were uplifted during the last great period of mountain-building—for it must be remembered that not all mountains are of the same age—and that the rocks of the earth had been folded into great arches, upon which were superinduced other folds, long before the uplift of the Himalayas. In other words, most of the highest mountains of the earth are much younger geologically than the older and less lofty ranges, just as the Rockies are vastly younger than the mountains of the Appalachian region, while they, in turn, are much younger than the Blue Ridge—itself younger than the remnants of other mountains which, however, in the attribute of elevation, are mountains no longer—since now only their upturned eroded roots remain marking sites where their masses formerly stood in the long distant past, as great, towering topographic features. Old mountains thus become bevelledbeveled [sic] off and their sediments may again, in the evolution of the continents, be uplifted elsewhere after eons of ages to form new and even much higher mountains than before.

The cause of the movements of the earth which produce mountains is a long story in itself—and so its discussion here must needs be omitted. For our present purpose it will suffice to say that such movements do occur. Furthermore, they proceed very slowly, so slowly that while such deformations are in progress they may be at such a rate as to be wholly imperceptible and yet, acting through a vast period of time, be sufficient to lift rock formations from beneath the sea to thousands of feet above sea level. In the course of the uplift, accompanied as it is by folding, obviously enormous stresses are developed and from time to time rocks give away and Assuring and faulting ensue; and when this happens the shocks that are felt are said to be the result of what is termed a tectonic earthquake. Later on other movements may occur along a line of weakness produced in this way, and at different points along this line and at different times, just as the California earthquake was the next to the last of a series of eight movements along a well defined line of crustal fracture. The movements which culminated in the California earthquake began in the north off the coast of British Columbia. The first and third of the series occurred in this region in September, 1899, and October, 1900. They were then felt off Central America in January, 1900, and April and September, 1902. After a lapse of over three years, sufficient stresses had accumulated to inaugurate another movement, upon the southern continuation of the same line. This shock was noted off southern South America on January 31, 1906; and then several months later came the California earthquake