I 



7 



Ch. xxiil] 



OF VOLCANIC REGIONS. 



f, 



~>95 



tended comparison, also, of the*history of the subterranean 



m 



ime 



time. It is impossible for us to declare, as yet, whether this 

 phenomenon is constant in this and other regions, because 

 we can rarely trace back a connected series of events farther 

 than a few centuries ; but it is well known that, where 

 numerous vents are clustered together within a small area, 

 as in many archipelagos for instance, two of them are never 

 in violent eruption at once. If the action of one becomes 



\ 



assume 

 not im 



pearance of spent volcanos. It is, therefore, 



that separate provinces of the same great range of volcanic 



fires may hold a relation to one deep-seated focus, analogous 



more superficial rent or cavity. 



some 



exam 



comparatively small distance from 



surface, Ischia and Vesuvius mutually communicate with 

 certain fissures, and that each affords relief alternately to 



elastic fluids and lava there generated. 



may 



Syria to be connected, at a much greater 



same svstem 



in which case any obstruction occurring in one duct may 

 have the effect of causing almost all the vapour and melted 

 matter to be forced up the other, and if they cannot get vent, 

 they may be the cause of violent earthquakes. Some objec- 

 tions advanced against this doctrine that 'volcanos act as 

 safety-valves,' will be considered in the sequel.* 



The north-eastern portion of Africa, including Egypt, 

 which lies six or seven degrees south of the volcanic line 

 already traced, has been almost always exempt from earth- 

 quakes; but the north-western portion, especially Fez and 

 Morocco, which fall within the line, suffer greatly from time 

 to time. The southern part of Spain also, and Portugal, have 

 generally been exposed to the same scourge simultaneously 



*!th Northern Africa. The provinces of Malaga, 7 



an <l Granada, and in Portugal the country round Lisbon, 

 ar e recorded at several periods to have been devastated by 



* See Ch, XXXII., Cause of Volcanic Eruptions. 



Murcia. 



Q Q k J 



<> 



