THEORY OF VOLCANOES. 97 



cules, which grow like the principal cone. If buds die, they are cov- 

 ered up in the annual layers of the trunk ; so, in like manner, extinct 

 monticules are buried in the layers of the principal cone. 



Estimate of the Age of Volcanoes. — The age of exogenous trees, as 

 is well known, may be estimated by counting the annual rings. The 

 age of yolcanoes can not be estimated accurately in a similar manner : 

 1. Because the overflows are not regularly periodical ; 2. Because in 

 the case of lava-overflows it requires many overflows to make one 

 complete layer ; and, 3. Because it is impossible to make a complete 

 section of the mountain. Nevertheless, Nature gives us partial sec- 

 tions, which reveal an almost incalculable antiquity. Thus, the Val de 

 Bove, of Etna (a huge valley reaching from near the summit to the 

 foot, and probably formed by an ingulf ment of a portion of the mount- 

 ain), gives a perpendicular section into the heart of the mountain 

 3,000 feet deep. Throughout the whole of this section the mountain 

 is composed entirely of layers of lava and cinders. It is almost cer- 

 tain, therefore, that the whole mountain to its very base, 11,000 feet, 

 is similarly composed. That the time necessary to accumulate this im- 

 mense pile, 11,000 feet high and ninety miles in circumference at the 

 base, was almost inconceivably great, is shown by the fact that Etna 

 had already attained very nearly its present size and shape 2,500 years 

 ago, when it was observed by the early Greek writers. The lava-stream 

 which stopped the Carthaginians in their march against Syracuse, 396 

 years before Christ, may still be seen at the surface, not yet covered by 

 subsequent eruptions. And yet Etna belongs to the most recent geo- 

 logical epoch, for it has broken through, and is built upon, the newer 

 tertiary strata. 



Theory of Volcanoes. 



In the theory of volcanoes there are two things to be accounted for, 

 viz. : 1. The force necessary to raise melted lava to the lips of the crater, 

 and even to project it with violence high into the air; 2. The heat 

 necessary to fuse rocks and form lava. 



Force. — The specific gravity of lava being about 2*5 to 3, it would 

 require the pressure of one atmosphere, or fifteen pounds to the square 

 inch, for every eleven or twelve feet of vertical elevation of the liquid 

 mass. The following table gives the pressure in atmospheres for four 

 well-known volcanoes, assuming the point of hydrostatic equilibrium 

 to be at the sea-level : 



NAME. 



Vesuvius . . 



Etna 



Mauna Loa 

 Cotopaxi . . 



Height. 



3,900 feet 

 11,000 " 

 13,800 " 

 19,660 " 



Pressure in atmospheres. 



325 



920 



1,150 



1,638 



1 



