GEOLOGY. 325 



times in a minute. As these globes burst, they scat- 

 ter the mud, sometimes to the amount of two or three 

 tons, from the centre. This mud is cold on the sur- 

 face, but said to be warm within. 



Extinct Volcanoes. 



By extinct volcanoes, is understood such as have 

 never, according to any human tradition or historical 

 record, been known to have been at any period in a 

 state of activity, yet whose products, still existing 

 demonstrate that they are of volcanic origin. The 

 most remarkable examples of these are in the pro- 

 vince of Auverge, in France.* 



Earthquakes. 



The frequency of earthquakes in volcanic countries, 

 and when they occur in non- volcanic countries remote 

 from volcanic fires, the coincidence of their occur- 

 rence with distant volcanic eruptions, strongly indi- 

 cate that they are influenced by the same cause. It 

 is a remarkable circumstance, also, that the shocks of 

 earthquakes are most severe in non-volcanic coun- 

 tries, such as Lisbon and the Caraccas. 



Source of Volcanic Fire. 



Water seems to be a necessary agent in the pro- 

 duction of volcanic fire, for only extinct volcanoes 

 are found far inland. The most active are in the 

 immediate vicinity of the sea, and some are actually 

 submarine. The matter that feeds them does not 

 seem to be universally diffused, but rather collected 

 in particular spots. Hence they almost always exist 

 in (groups, yet the action of one of the volcanoes of 

 the same group is found to be completely independent 



* Homer says nothing about the eruption; 

 historian that mentions them is Thucydides. 



tions of Etna j the earliest 



