144 DYNAMICAL GEOLOGY. 



ate in our estimate is shown by the fact that there are 

 known on the flanks of Etna lava-flows 2,000 years old, 

 which are still not covered by subsequent flows. We are 

 justified, then, in saying that Etna is probably much 

 more than 300,000 years old. But the birth of Etna is a 

 very recent geological event, for it stands, and has been 

 built up, on the latest tertiary formation. 



Cause of Volcanic Eruptions. 



ds question is still very obscure. There are two 

 things to be explained, viz., volcanic force and volcanic 

 heat the force necessary to raise lava to the lip of the 

 crater, and the heat necessary to melt the lava. 



(a.) Force. -If we consider the height of volcanic 

 cones, we shall be better able to appreciate the greatness 

 of the force. In the accompanying table we give the 

 heights of some well-known volcanoes and the pressure in 

 atmospheres (one atmosphere = fifteen pounds per square 

 inch, or one ton per square foot) necessary to raise lava 

 (taking the specific gravity at 2.8) to the lip of the crater. 

 It is true lava is sometimes foamy, and therefore lighter, 

 but, on the other hand, we have taken the focus of vol- 

 canoes at sea-level, while it is probably much deeper, and 

 have supposed the force only sufficient to raise to the lip 

 of the crater, whereas it often ejects with violence many 

 thousand feet in the air. 



What, then, is the agent of this great .force ? It is 

 believed that it is the elastic force of compressed gases 



