Page:Popular Science Monthly Volume 27.djvu/851

Rh masses as we have just been considering. It is hard for the mind to admit two causes so different for phenomena which in most of their features so resemble one another. The demarkation between them is hard to define. On the western coast of South America and in Venezuela earthquakes facing volcanic ranges and those distant from such phenomena present the same manifestations. The supposition that the phenomena are due to the rubbing together of the solid parts of the crust also encounters a serious objection in the remarkable repetition of shocks during the same crises. In fact, this reiteration of shocks by the hundred and the thousand, for weeks and months, is one of the most characteristic accompaniments of earthquakes.

In view of these periods of disturbances, the cause, instead of being exhausted by a few shocks at short intervals, as would happen under the supposition that the first cause is the action of solid masses upon one another, should be one that could re-enforce itself after having been temporarily weakened. This is an essential fact, which any proposed solution must explain. We first remark that water, confined in a close space which it fills, will come, when it reaches a high enough temperature, to have a power which it is hard to represent in figures.

In nature, the tension of the vapor in the volcanic reservoirs is exhibiting its energy at every instant; for that which forces the lava out of the crater of Etna, about ten thousand feet above the sea, can not be less than one thousand atmospheres. The conditions necessary to give such tensions can not fail to be realized in the crust of the earth at a certain depth, beyond the domains of real volcanoes, and principally under chains of mountains and dislocated tracts. It is an ascertained fact that, whatever may be the constitution of the ground, the temperature increases regularly as we descend to a lower depth. At the same time water tends to descend, under the joint influence of gravity and capillarity, and may continue to descend till it reaches the deep and hot regions where it can acquire a temperature that will render it capable of producing great mechanical and chemical effects. Hence we can hardly doubt that waters from the surface reach the internal regions, and then make us feel in the shape of tremors and rumbling the power and explosive force which they gain there.

The depth at which the agency from which earthquakes originate should be sought has been the subject of careful studies. The results indicate that it is not seated in the central parts of the globe. This is the inevitable conclusion from such cases as the earthquakes in Calabria, in which the disturbed area was very small.

It is probable that the consolidation of the deep parts under the dislocated regions and especially under chains of mountains of a relatively recent age is not yet completed, and that there are still interstices and interior cavities of high temperature which eventually became filled with water by the action of capillarity. Hence we find the