246 COSMOS. 



subterranean fire, and that littoral situations only favor the 

 eruption by forming the margin of a deep sea basin, which, 

 covered by strata of water, and lying many thousand feet lower 

 than the interior continent, can offer but an inconsiderable 

 degree of resistance. 



The present active volcanoes, which communicate by per- 

 manent craters simultaneously with the interior of the earth 

 and with the atmosphere, must have been formed at a subse- 

 quent period, when the upper chalk strata and all the tertiary 

 formations were already present : this is shown to be the fact 

 by the trachytic and basaltic eruptions which frequently form 

 the walls of the crater of elevation. Melaphyres extend to the 

 middle tertiary formations, but are found already in the Jura 

 limestone, where they break through the variegated sandstone.* 

 We must not confound the earlier outpourings of granite, quartz- 

 ose porphyry, and euphotide from temporary fissures in the old 

 transition rocks with the present active volcanic craters. 



The extinction of volcanic activity is either only partial — 

 m which case the subterranean fire seeks another passage of 

 escape in the same mountain chain — or it is total, as in Au- 

 vergne. More recent examples are recorded in historical times, 

 of the total extinction of the volcano of Mosychlos,t on the 

 island sacred to HephsBstos (Vulcan), whose " high whirling 

 flames" were known to Sophocles ; and of the volcano of Me- 

 dina, which, according to Burckhardt, still continued to pour 

 out a stream of lava on the 2d of November, 1276. Every 

 stage of volcanic activity, from its first origin to its extinction, 

 is characterized by peculiar products ; first by ignited scoriae, 

 streams of lava consisting of trachyte, pyroxene, and obsidian, 

 and by rapilli and tufaceous ashes, accompanied by the devel- 



* Dufreuoy et Elie de Beaumont, Explication de la Carte Giologiqtie 

 de la France, t. i., p. 89. 



t Sophocl.,PA.z7oc^., V. 971 and 972. On the supposed epoch of the 

 extinction of the Lemnian fire in the time of Alexander, compare Butt- 

 mann, in the Museum der Alterthumswissenschaft, bd. i., 1807, s. 295 ; 

 Dureau de la Malle, in Malte-Brun, Annales des Voyages, t. ix., 1809, 

 p. 5 ; Ukert, in Bevtuch, Geogr. Epkemeriden, bd. xxxix., 1812, s. 361 ; 

 Rhode, Res Lemnicce, 1829, p. 8 ; and Walter, Ueber Abnahme der Vul- 

 lean. Thaiigkeit in HistoriscJien Zeiten, 1844, 8 24. The chart of Lem- 

 nos, constructed by Choiseul, makes it extremely probable that the ex- 

 tinct crater of Mosychlos, and the island of Chryse, the desert habitation 

 of Philoctetes (Otfried Mliller, Minyer, s. 300), have been long swal- 

 lowed up by the sea. Reefs and shoals, to the northeast of Lemnos, 

 Btill indicate the spot where the .^gean Sea once possessed an active 

 volcano like iEtna, Vesuvius, Stromboli, and Volcano (in the Lipari 

 Isles). 



