A STUDY OF RECENT EARTHQUAKES. 829 



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 Vene- 

 zuela 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 hav- 

 ing been temporarily weakened. This is an essential fact, which any 

 proposed solution must explain. We first remark that water, con- 

 fined 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 Cala- 

 bria, 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 rela- 

 tively recent age is not yet completed, and that there are still inter- 

 stices and interior cavities of high temperature which eventually be- 

 came filled with water by the action of capillarity. Hence we find the 



