RELATION OF SEISMIC TO VOLCANIC PHENOMENA. 275 



They are due to tlie explosive action of steam bursting 

 through the molten lava. 



Volcanic eruption succeeding earthquakes, — Some- 

 times has happened that an earthquake, or a series of 

 earthquakes, have terminated with the formation of vol- 

 canic vents. 



As an example of a volcanic outburst terminating a 

 seismic disturbance, may be mentioned the appearance 

 of a new volcano in the centre of Lake Ilopango, as a 

 sequel to the shocks which had disturbed that neigh- 

 bourhood in 1879.^ 



In 1750 there were continuous shakings lasting over 

 three months at Manilla. These terminated with an 

 eruption of a small island in the middle of a neigh- 

 bouring lake. Three days after the commencement of 

 this eruption, four other small islands rose in the same 

 lake.2 



Antonio d'Ulloa, when speaking of the Andes, remarlis 

 that after a volcanic eruption the shocks cease.^ 



Conclusion. — Looking at this question generally, in- 

 somuch as the greatest number of volcanic eruptions 

 appear, according to Fuchs, to have taken place in summer, 

 whilst the greatest number of its earthquakes have 

 apparently taken place in winter, it would seem that 

 the two phenomena are without any direct connection, 

 unless it be that both are different effects of a common 

 cause. 



Eegarded in this manner, an earthquake may be looked 

 upon as an uncompleted effort to establish a volcano. To 

 use the words of Mallet, ' The forces of explosion and 

 impulse are the same in both ; they differ only in degree 



» * Earthquakes of San Salvador, December 21-30, 1879.' Am. Jour. 

 Sci. vol. xix. p. 415. 



2 Gent. Mag. 1757, p. 323. » PJul. Tran.<i. vol. li., 17G0. 



