ON ITS EXTERIOR. VOLCANOES. 221 



such explosion is regularly announced by small and purely 

 local earthquake shocks. Sometimes lava is poured out 

 from the open fissures or hollows, but without making its 

 way beyond the sides of the crater; and when it does 

 break through them, the new stream of molten rock 

 usually finds a course which does not prevent the great 

 crater-valley itself from being accessible even during such 

 minor eruptions. The margins of craters appear to undergo 

 far less variation than might have been expected; for ex- 

 ample, in the case of Vesuvius, Saussure's measurements 

 compared with mine, shew that no change beyond the limits 

 of observation error took place in the height of the north- 

 western edge of the volcano, the E-occa del Palo, in the 

 interval of forty-nine years, from 1773 to 1822. (9W) I 

 have been solicitous to give an accurate idea of the form and 

 normal structure of volcanoes, without which it is impos- 

 sible to attain a right understanding of phsenomena, which 

 have been long much disfigured by fanciful descriptions, 

 and by the equivocal and ill-defined use of the terms, 

 crater, cone of eruption, and volcano. 



Volcanoes which, like those of the Andes, rise high above 

 the region of perpetual snow, present peculiar phsenomena; 

 the masses of snow, by their sudden melting during 

 eruptions, produce terrible inundations and torrents of 

 water, by which smoking scoriae are hurried along with 

 blocks of k-e ; they also exert a continued action during 

 the periods when the volcano is in a state of entire repose, 

 by infiltration into the fissures of the trachytic rocks. Ca- 

 vities in the declivity or at the foot of the volcano are thus 

 gradually converted into subterranean reservoirs of water, 

 with which the alpine torrents and rivulets of the highlands 



