TRUE VOLCANOES. 265 



feet above the walls of the crater. Gemellaro's estmates 

 during the eruption of Etna in 1832. gave even three 

 times this elevation ! The black, erupted ashes form layers 

 of three or four hundred feet in thickness upon the decli- 

 vities of the Sari ofay for a circle of nearly fourteen miles in 

 circumference. The colour of the ashes and rapilli gives the 

 upper part of the cone a fearfully stern character. We must 

 here again call attention to the colossal size of this volcano, 

 which is six times greater than that of Stromboli, as this 

 consideration is strongly in opposition to the absolute belief 

 that the lower volcanoes always have the most frequent 

 eruptions. 



The grouping of volcanoes is of more importance than 

 their form and elevation, because it relates to the great 

 geological phenomenon of upheaval upon fissures. These 

 groups, whether according to Leopold von Buch, they rise in 

 lines, or united around a central volcano, indicate the parts 

 of the crust of the earth, where the eruption of the fused 

 interior has found the least resistance, in consequence either 

 of the reduced thickness of the rocky strata, of their natural 

 structure, or of their having been originally fissured. Three 

 degrees of latitude are occupied by the space in which the 

 volcanic energy is formidably manifested in Etna, in the 

 ^Eolian Islands, in Vesuvius, and the parched land (the Phle- 

 grsean Fields) from Puteoli (Dicearchia) to Cumse, and as far 

 as the fire- vomiting Epopeus on Ischia, the Tyrrhenian island 

 of Apes, ^Enaria. Such a connexion of analogous phenomena 

 could not escape the notice of the Greeks. Strabo says, " The 

 whole sea commencing from Cumse as far as Sicily is pene- 

 trated by fire, and has in its depths certain conduits commu- 

 nicating with each other and with the continent. 61 In such a 



61 See Strabo, lib. v, p. 248, Casanbou : t\ti KoiXmc Tirdc; and 

 lib. vi, p. 276. Upon a double mode of production of islands the 

 geographer of Amasia expresses himself (vi, p. 258) with much geolo- 

 gical acumen. " Some islands," says he (and he names them), " are 

 fragments of the mainland ; others have proceeded from the sea, as still 

 happens. For the islands of the high sea (those which lie far out in 

 the sea) were probably upheaved from the depths ; whilst, on the con- 

 trary, it is more reasonable to consider those situated at promontories 

 and separated by a strait, as torn from the mainland." The small group 

 of the Pithecusae consists of Ischia, originally called ^Enaria, and Procida 

 (Prochyta). The reason why this group was considered to be an ancient 

 habitation of apes, why the Greeks and the Italian Tyrrhenians, conse- 



