The Evolution of the Sciences 



the disappearance of the island of Ustica near 

 Sicily, and the eruption of Vesuvius. 



Now one single observation of this kind is 

 sufficient to establish the common origin of 

 several volcanoes; on the other hand, it stands 

 to reason that the volcanoes of the same region 

 should in general be independent. If any cause 

 happens to disturb the equilibrium of the internal 

 mass the increased pressure will fracture the 

 crust at the point of least resistance, and the 

 entire volcanic activity will concentrate in this 

 crack. This is why, in times of paroxysm, the 

 same regions, like ill-healed wounds, form a vent 

 for the internal flux. However, when the convul- 

 sions of the crust produce a new line of fracture, 

 or the hardened lavas choke up the old vent, the 

 next eruption will take another direction. 



Therefore one must not be surprised to see 

 very rarely neighbouring volcanoes in sim- 

 ultaneous activity. Neither must one draw 

 conclusions from the difference of the products 

 erupted, because one identical volcano is found 

 to supply products of the most varied nature. 

 At Teneriffe the upper lavas contain over 



58 per cent, of silica, the intermediate ones contain 



146 



