Page:Popular Science Monthly Volume 27.djvu/848

826 an explanatory hypothesis. The phenomena have been variously ascribed to subterranean electric storms, to the influence of the sun, supposed to be potent over the interior regimen of the planets as well as upon their course in their orbits; to thrusts of the liquid or semi-liquid masses of the interior against parts of the solid crusts, which may be caused by the same forces as produce the tides; to sudden reductions of atmospheric pressure; or to the fall of immense masses of rocks in vast interior cavities.

Numerous and exact studies, bringing into clear view the relations of earthquakes with the geological structure of the countries subject to them, have given us a better comprehension of their organic causes. An important fact, developed by patient statistical research, is the great inequality in the geographical distribution of the phenomena. There are vast regions in which they are very rare and feeble, and others where the agitations are frequent and often very violent. But it is a significant fact in this connection that the frequency of the disturbances is not so much associated with geographical position as with peculiar characters in the constitution of the crust of the earth. Thus, many earthquake regions are characterized by the presence of active volcanoes. A striking example of such association is presented in the narrow tract between the Andes and the Pacific Ocean, in South America, particularly in Colombia, Ecuador, and Chili. "On the coasts of Peru," says A. von Humboldt, "the sky is always clear; neither hail nor stones nor fierce lightnings are known; the subterranean thunder attending the earthquake-shocks takes the place of the thunder of the clouds. By long habit and the general opinion that only two or three destructive shocks are likely to occur in a hundred years, the people of Lima are but little more afraid of earthquakes than those of the temperate zones are of hail-storms." In this region, between the sixteenth and twenty-fourth degrees of latitude, there are eighteen volcanoes; Chili, eminently subject to earthquakes, has thirty-three active volcanoes, between 33° and 43° south. Very different conditions prevail east of the Cordilleras, where vast countries like Brazil have no earthquakes. Farther north, on the isthmus, there are regions where the shocks are so frequent that one of them has been called "Cuscuttan," or the hammock. The single state of Nicaragua has twenty-four volcanoes. Along the coast of Asia is a zone of volcanoes and earthquakes about 9,000 miles long. It begins at Barren Island in the Bay of Bengal, crosses Sumatra, Java, the Moluccas, and the Philippines, bends around by Formosa and the neighboring archipelagoes to Japan, and then to the Kurile Islands and Kamchatka, and ends at last in the Aleutian Islands. Through all this zone the volcanoes are numerous and active, and in some parts of it at least, as in Japan and the Philippine Islands, the earth is never at rest. The seismograph at Manila is always in motion, even when the ground seems still, and a year never passes without a severe shock. The connection