A STUDY OF RECENT EARTHQUAKES. 827 



between earthquake crises and volcanic crises is also shown by alterna- 

 tions in their activity. Every volcanic eruption is heralded by pre- 

 cursory tremors, whose violence is calmed down as soon as a volcanic 

 outlet is opened for the escape of the vapors. Now, the vapor of 

 water is the recognized cause of volcanic eruptions, and constitutes, iu 

 all parts of the earth, the most abundant and most constant emanation 

 from them. It is the agent that throws out from the depths to the 

 surface the lavas, which, despite their high temperature, hold it in- 

 corporated in their paste ; in the same way that carbonic acid, dis- 

 solved in water, forces the liquid impetuously out of a mineral-water 

 or a champagne bottle ; and it also shoots quantities of solid matter, 

 stones, lapilli, and cinders, violently into the atmosphere. It is logical 

 to believe that this vapor is likewise the cause of the agitations that 

 accompany volcanic crises. Agreeably to this idea, Kircher and Hum- 

 boldt regarded volcanoes as safety-valves against earthquakes. 



Other countries, again, where there are no volcanoes, are disturbed 

 with no less energy and frequency, and that over great areas. Of 

 such is the southern part of the basin of the Mediterranean. Syria 

 with Palestine, Asia Minor, Turkey in Europe, Greece and the archi- 

 pelagoes, Italy, Sicily, the southern part of the Iberian Peninsula, and 

 ,a part of its western coast around Lisbon, have shown evidences of 

 this predisposition within historical times. In each of these countries 

 are districts or places that have been associated with most disastrous 

 convulsions. In them we may discern a common and essential charac- 

 teristic in the shape of a dislocation of the constituent strata, which is 

 revealed for the most part in a mountainous relief. 



In some whole countries the sedimentary strata, which form a nota- 

 ble part of the thickness of the earth's crust, have remained horizontal 

 or nearly so, as they were deposited. In other countries, and over 

 considerable areas, the corresponding strata are raised up, bent, and 

 contorted in different ways, having been subjected to dislocations 

 through enormous thicknesses. Such lifts and foldings can not have 

 taken place in solid masses without being accompanied by many and 

 important fractures. The principal classes of such fractures, which 

 are nearly vertical, are called faults. They crop out and cut the sur- 

 face of the ground, sometimes for tens and hundreds of miles, and are 

 of indefinite depth, or descend to below where it is possible for man 

 to penetrate ; whenever a fault is produced, the two sides are displaced, 

 and must rub hard upon one another ; and vast rocky surfaces are 

 thus engraved, striated, and polished : thence they are called mirrors in 

 the language of the miners. 



The occurrence of these facts is not confined to mountain-chains, 

 but may be observed in countries that are marked by only slight 

 prominences, but which have undergone similar actions through all 

 their constituent strata. It is evident that the solid envelope of the 

 globe has undergone dislocations at many epochs in its history. The 



