BOOK IV. 



VOLCANOES AND EARTHQUAKES. 



AFTER the mantle of snow which envelops the summits of 

 the mountains, that which strikes us most is their volcanoes. 

 Seen from afar, they only give a very imperfect idea of 

 what they are. To appreciate their phenomena and their 

 ravages, our eyes must survey their depths. All is then 

 changed, and the grandeur of the spectacle strikes the imag- 

 ination, graving terrible images on it. We are astonished 

 at the immensity of their fire-spouting mouths, and at the 

 vastness of the lava streams which flow from them at cer- 

 tain times. 1 



The mythology of Greece gave to these mountains an 



1 Some men of science have expressed their wonder that the interior of the 

 earth can furnish matter sufficient for these eruptions, but a little reflection will 

 show that no great contraction of the crust of the globe is required to feed them. 

 Violent eruptions do not usually emit more than 1300 cubic yards of lava, and 

 seldom so much. This quantity, supposing it spread equally over the surface of 

 the globe, would not form a layer so much as -^th of a millimetre (or about 

 T fi^th of an inch) in thickness. Thus we see that a contraction of the earth 

 sufficient to shorten its radius by one millimetre would furnish matter for five 

 hundred violent eruptions ; and on consulting the history of recent volcanic phe- 

 nomena we arrive at the conclusion that a contraction of l inch is sufficient to 

 have supplied the lava thrown up in all the eruptions that have occurred on our 

 planet during the last 3000 years. 



