Page:Popular Science Monthly Volume 29.djvu/319

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E are accustomed to think of the land of the earth as something solid and fixed; and, as a testimonial of this impression, the Latin phrase terra firma, firm land or solid ground, has been naturalized in the languages of nearly all civilized peoples. On the other hand, we speak of water as unstable. But the geological history of the earth and the more careful observations of modern times have taught us that these ideas do not correctly represent the qualities of the land-masses and water-masses of the globe as compared with one another. The ancient shore-marks on the continents and the phenomena of elevation and subsidence that have been observed in historic times, confirming their evidence, show that the land and the ocean are continually changing their level as to one another; and it has further been made evident, by experiment, as well as by a priori reasoning, that it is not the ocean that changes, but the land which undergoes alternate movements of elevation and depression. An earthquake-shock is a phenomenon well adapted to destroy the faith of any person who feels one in the fixedness of the earth; and such, by the evidence, is the effect for the time on all who experience these shocks. Even the light pulsations which sometimes pass over parts of the United States occasion panic and excite a momentary impression that everything is falling over or sinking away; while the more violent shocks that are felt in earthquake-infested countries produce indescribable terror; and such catastrophes as those historical earthquakes of Lisbon and Caracas, and the more recent ones of Ischia and the Strait of Sunda amount to a demonstration that the reason for such terrors is real, and that the continents also can not escape the general law of change and perishability.

Earth-movements—the name by which these phenomena may be most conveniently described are various, and comprise, so far as they are now considered, earthquakes, or sudden violent movements of the ground; earth-tremors, or minute movements which usually escape attention by the smallness of their amplitude; earth pulsations, or movements which are overlooked on account of the length of their period; and earth oscillations or movements of long period and large amplitude—like the shifting of levels of land-masses—which attract attention from their geological importance. Some of these movements have only recently begun to attract attention. They are all intimately associated in their occurrence and their origin.

The study of earthquakes is of interest to the geologist in many