BOOK lY. 



VOLCANOES AND EAETHQUAKES. 



After the mantle of snow wliicli overspreads tlie moun- 

 tains, that which strikes ns most is their volcanoes. Seen 

 from afar they only give a very imperfect idea of ^^diat they 

 are. To appreciate their phenomena and their ravages, 

 our eyes nmst survey their depths. All is then changed, 

 and the grandeur of the spectacle strikes the imagination, 

 graving terrible images on it. We are astonished at the 

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

 ness of the lava streams which flow from them at certain 

 times.^ 



The loftier volcanoes are, the less frec|uent are their 

 eruptions. The lava which they vomit forth, issuing from 



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

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

 show that no great contraction of the crust of the globe is requiretl 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 --i-ijth of a millimetre (or about 

 -niFTnith of an inch) in thickness. Thus we see that a contraction of the earth sufli- 

 cient to shorten its radius by one millimetre, would furnish matter for five hundred 

 violent eruptions; and on consulting the history of recent volcanic phenomena 

 we arrive at the conclusion that a contraction of .3 centimetres (or ]i 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. 



