224 EARTHQUAKES. 



If the earthquake originated by the formation of a 

 fissure, after the rending has gone on for a certain time 

 the focal cavity is enlarged to a certain extent, and the 

 great shock takes place. This would be followed by con- 

 cluding tremulous waves. A succession of phenomena 

 like those accompanied the shock about which Mallet 

 writes. 



By observations such as these, coupled with what has 

 been said about the maximum and mean depths of the 

 focal cavity, Mallet came to the conclusion that the focal 

 cavity was a fissure, the rending open of which had 

 produced the earthquake. The vertical dimensions of 

 this cavity were not more than 5*3 miles, but were 

 probably limited to three miles. 



From the intersection of the wave paths upon the 

 surface and the observed emergences, this fissure fol- 

 lowed horizontally a curve of double flexure, about nine 

 geographical miles in length. The area of this fissure was 

 twenty-seven geographical miles. The time of rending it 

 open in Apennine limestone would be about 7J- seconds, 

 which should be the same as the period during which 

 tremors were felt. The time actually recorded was six 

 or eight seconds. 



Briefly, this is, then, the line of reasoning which was 

 followed by Mallet in an investigation the results of 

 which are as interesting as they are startling. Since the 

 line of investigation has been opened, and the existence 

 of new problems has been indicated, other investigators, 

 although not exactly following Mallet's method in all 

 their details, have, when endeavouring to attain the same 

 ends, employed similar weapons. 



Thus, for example, Seebach, when determining the 

 depth and nature of the origin of the earthquake of 

 Middle Grermany, reasoned somewhat as follows : — 



