EARTHQUAKES. 367 



with perfect uniformity all round. Thus, one part of the surface will 

 go down before another, and as this must usually occur by a cracking 

 and sudden motion, the result will be an earthquake. 



The seismic ribbons of which we have spoken are probably lines of 

 weakness along which cracking habitually takes place. Along these 

 lines there are enormous dislocations of the geological strata, and 

 earthquakes are known to follow lines of faulting. The geologically 

 recent elevation of the Pacific coast of South America is obviously, 

 from this point of view, connected with the abundance of volcanoes 

 and the frequency of earthquakes along the chain of the Andes. 



One would think that the continual ejection of lava and ashes 

 from an active volcano must leave a hollow under the mountain, and 

 that some day the cavern would suddenly collapse. It has, however, 

 been observed that volcanic eruptions and severe earthquakes are to 

 some extent alternatives, so that it seems as though the volcanic vent 

 served as a safety-valve for the liberation of the dangerous matter. 

 But the theory of the collapsing cavern must not be wholly rejected, 

 for some disastrous earthquakes affecting only very restricted areas, 

 such as that of Casamicciola in Ischia, are hardly otherwise explica- 

 ble. In this case Palmieri has attributed the formation of the cavern 

 to evisceration under the town produced by hot mineral springs. 



In the theories of which we have just spoken, the internal heat of 

 the earth acts indirectly, by giving to gravitation an opportunity of 

 coming into play. But as in volcanic eruptions enormous quantities 

 of steam are usually emitted, it is probable that the pressure of steam 

 is the force by which the lava and ashes are vomited forth, and that 

 the steam is generated when water has got among hot internal rocks. 

 From this point of view we can understand that an eruption will serve 

 as a protection against earthquakes, and that the centers of disturbance 

 will usually be submarine. 



It may on the whole be safely concluded that a diversity of causes 

 are operative, and that some earthquakes are due to one and others to 

 other causes. 



It would, however, be certainly wrong to look only to the interior 

 of the earth for the causation of earthquakes, since the statistics of 

 earthquakes clearly point to connections with processes external to the 

 solid earth. 



The laborious inquiries of M. Perrey show that there are more 

 earthquakes at the time of full and change of moon than at other 

 times, more when the moon is nearest to the earth and more when she 

 is on the meridian than at the times and seasons when she is not in 

 those positions relatively to the earth. The excess of earthquakes at 

 these times is, however, not great, and an independent investigation 

 of the Japanese earthquakes does not confirm Perrey's results. It is 

 well, therefore, still to hold opinion in suspense on this point. If, 

 however, Perrey's result should be confirmed, we must attribute it to 



