280 EAETHQUAKES. 



out from the cooling interior of our globe, its sudden 

 explosion might be brought about by its own expansive 

 force, or by the movements in the bubbling mass from 

 which it originated. 



Others, however, rather than regard the steam as being 

 a primeval constituent of the earth's interior, imagine it 

 arises from the gradual percolation of water from the 

 surface of the earth down to volcanic foci, into which it is 

 admitted against opposing pressures, by virtue of capillary 

 action. 



Mallet, in his account of the Neapolitan earthquake, 

 shows that the whole of the observed phenomena can be 

 accounted for by the admission of steam into a fissure, 

 which by the expansive force exerted on its walls was 

 rent open. Just as at the Greysers we hear the thud and 

 feel the trembling produced by the sudden evolution and 

 condensation of steam, so may steam by its sudden 

 evolution and condensation in the ground beneath us 

 give rise to a series of shocks of varying intensity, accom- 

 panied by intermediate vibratory motions — that is to say, 

 a motion which, as judged of by our feelings, is not unlike 

 many earthquakes. Often it may happen that the result 

 of the explosion may be the production of a fault, or at 

 least a fissure ; and thus in the resulting movements we 

 may have a variety of vibrations, some being those of 

 compression and distortion, produced by the blow of the 

 explosion, and others being those of distortion alone, 

 produced by the shearing action which may have taken 

 place by the opening of the fault. Sometimes one set of 

 these vibrations may be prominent, and sometimes the 

 other. Thus, when we say that an earthquake has shown 

 evidence by the nature of its vibrations that it was pro- 

 duced by a fault, this by no means precludes the possi- 

 bility that an explosion of steam may also have been con- 



