430 COSMOS. 



of seeing specimens of this pumice-stone in the interesting 

 geological collections of my learned friend and academical 

 colleague, Dr. Ewald. The similarity of the mineralogical 

 constitution at two opposite points naturally gives rise to 

 the question, whether that which covers Pompeii has been 

 thrown down, as Leopold vcn Buch supposes, during the 

 eruption of the year 79, from the declivities of Somma, or 

 whether, as Scacchi maintains, the newly-opened crater of 

 Vesuvius has ejected pumice simultaneously on Pompeii aiid 

 on Somma? What was known as pumex Pompejanus in 

 the time of Vitruvius, under Augustus ,carries us back to 

 eruptions before the time of Pliny, and from the experience 

 we have respecting the variable nature of the formations in 

 different ages and different circumstances of volcanic activity, 

 we should be as little warranted in absolutely denying that, 

 since its first existence, Vesuvius could have ejected pumice, 

 as we should be in absolutely taking it for granted that 

 pumice that is to say, the fibrous or porous condition of a 

 pyrogenous mineral could only be formed where obsidian 

 or trachyte with vitreous felspar (sanidine) were present. 



Although, from the examples which have been cited of the 

 length of the periods at which the revival of a slumbering 

 volcano may take place, it is evident that much uncertainty 

 must still remain, yet it is of great importance to verify 

 the geographical distribution of burning volcanoes for a 

 determinate period. Of the 225 open craters through which, 

 in the middle of the 19th century, the molten interior of 

 the earth maintains a volcanic communication with the 

 atmosphere, 70, that is to say, one-third, are situated on the 

 continents, and 155, or two-thirds, on the islands of our 

 globe. Of the 70 continental volcanoes, 53, or three-fourths, 

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

 that portion of the continent of Africa hitherto known to us. 

 In the South-Asiatic islands (the Sundas and Moluccas), as 

 well as in the Aleutian and Kurile Islands, the greatest num- 

 ber of the island volcanoes are situated in a very limited 

 space. The Aleutian Isles contain, perhaps, more volcanoes 

 active in late historical times than the whole continent of 

 South-America. On the whole surface of the earth, the tract 

 containing the greatest number of volcanoes is that which 

 ranges between 73 west and 127 east longitude, and 



