4 



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oin 



h 



V 



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-*a 



its of 



1 



^^^1 of huge 



itiiiuoTis line 



■ '^' Great 

 of the 



-t Avliere the 



•'^ *:iken in 



re species, as 

 in the sand, 



>1 quantity. 



n'^l to have 

 rapours from 



r S nia tad 



]^,ijl2- iVtS to 



of the people 

 bin S?^ 



Lilt h>^' 



i.and 



lain, i;; 



V 



orcr 



it, a»^ 



1 



1. 



but 



sooo 





-iv 



At 



W 



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1 7S3. 



iV 



5, 



Ch. XXIX.] 



ORIGIN OF EARTHCIUAKE-WAVES 



135 



when the first great shock afflicted Calabria, the volcano of 

 Stromboli, which is in full view of that town, and at the 



ini 



qnantity of inflamed matter, than it had done for some years 

 previously. On the other hand, the great crater of Etna is 

 said to have given out a considerable quantity of vapour 

 towards the beginning, and Stromboli towards the close, of 



commotion 



from 



of these great vents during the whole earthquake, the sources 

 of the Calabrian convulsions, and of the volcanic fires of Etna 

 and Stromboli, appear to be very independent of each other ; 

 unless, indeed, they have the same mutual relation as Vesuvius 

 and the volcanos of the Phlegrsean Fields and Ischia, a violent 

 disturbance in one district serving as a safety-valve to the 

 other, and both never being in full activity at once. 



}f propagation of 



XXIII 



We 



■ood 



reasons for suspecting that the subterranean causes of the 

 earthquake and volcano are the same. In what manner 

 Tinrf,ioTis of the sohd crust of the earth may be melted from 



time 



time so 



form 



of fused matter at 



various depths, will be considered in Chapter XXXII. 

 Assuming for the present the existence of such reservoirs of 

 liquid lava in the interior, it is not difficult to understand 

 how steam may be generated whenever rain-water or the 

 waters of the sea, percolating through rocks, gain access to 

 snp"h lava,, and how when steam is generated, the incumbent 



crust of the earth may be rent and dislocatea. 



During such movements fissures may be formed and 

 injected with gaseous or fluid matter, which may sometimes 

 fail to reach the surface, while at other times it may be 

 expelled through volcanic vents, stufas and hot springs. 

 When the strain on the rocks has caused them to split, or 

 the roofs of pre-existing fissures or caverns have been made 

 to fall in, vibratory jars will be produced and propagated in 

 all directions, like waves of sound through the crust of the 

 earth with varying velocity, according to the violence of the 

 original shock, and the density or elasticity of the substances 

 through which they pass. They will travel, for example. 



