410 REACTION OF THE INTERIOR OF THE EARTH 



overspread Pompeii was, as Leopold von Buch would 

 consider, thrown down from the declivities of the Somma 

 in the eruption of 79, or whether, as Scacchi maintains, 

 the newly opened crater of Vesuvius cast forth pumice 

 simultaneously on Pompeii and on the Somma? The fact 

 of "pumex Pompeianus" being known to Vitruvius in the 

 time of Augustus, points to eruptions anterior to Pliny : 

 and from the experience which we have had of varia- 

 bility in the formations by volcanic activity in different 

 ages and under different circumstances, we are by no 

 means warranted either in absolutely denying the possi- 

 bility of Vesuvius having ever at any former period pro- 

 duced pumice, or in absolutely assuming that pumice, 

 i. e. a fibrous or porous state of a pyrogenous mineral, can 

 only be formed where obsidian or trachyte is present 

 with glassy felspar (sanidine). 



Although, according to the examples adduced, much 

 uncertainty still subsists as to the length of the periods 

 after, which a slumbering volcano may re-awaken, yet it 

 is of great importance to ascertain the geographical 

 distribution of burning volcanoes at a determinate 

 epoch. Of the 225 orifices through which, in the 

 middle of the nineteenth century, the molten interior of 

 the earth is in volcanic communication with the atmo- 

 sphere, 70 (less than a third part therefore) are on con- 

 tinents, and 155 (or fully two thirds) are on islands. Of 

 the 70 continental volcanoes, 53 (or three fourths) belong 

 to America, 15 to Asia, 1 to Europe, and 1 or 2 to the 

 portion of Africa with which we are acquainted. It is 

 in the South-Asiatic Islands (the Sunda isles and the 

 Moluccas) and in the Aleutian and Kurile (East- Asiatic) 

 islands, that the greatest number of island volcanoes 

 are congregated within the smallest space. The Aleu- 



