CRATERS OF ELEVATION. 227 



to say, a ci-ater-like, round or oval depression, bounded by a 

 circle of elevation, a ring-shaped wall, usually broken down 

 in places ; sometimes (when the framework of a perma- 

 nent volcano is to be completed), to a dome-shaped or 

 conical mountain in the middle of the crater of elevation. 

 The latter is then generally open at its summit, and on the 

 bottom of this opening (the crater of the permanent volcano) 

 rise transitory hills of eruption and hills of scoriae, small and 

 large cones of eruption, which, in Vesuvius, sometimes far ex- 

 ceed the margins of the crater of the cone of elevation. The 

 signs of the first eruption, the old framework, are not however 

 always retained. The high wall of rock which surrounds 

 the inner circular wall (the crater of elevation), is not recog- 



and Physicalische Beschreibung der canarischen Inseln, 1825, s. 213, 

 262, 284, 313, 323, and 341. This work, which constitutes an era in 

 the profound knowledge of volcanic phenomena, is the fruit of a voyage 

 to Madeira and Teneriffe from the beginning of April to the end of 

 October, 1815 ; but Naumann indicates with much justice, in his Lekr- 

 buch der Geognosie, that in the letters written in 1802 by Leopold von 

 Buch, from Auvergne (Geognostische Beobacht. auf Reisen durch Deutsch- 

 land und Italien, Bd. ii, s. 282), in reference to the description of Mont 

 d'Or, the theory of craters of elevation and their essential difference 

 from the true volcanoes was already expressed. An instructive coun- 

 terpart to the three craters of elevation of the Canary Islands (on Gran 

 Canaria, Teneriffe, and Palma) is furnished by the Azores. The admir- 

 able maps of Captain Vidal, for the publication of which we are in- 

 debted to the English Admiralty, elucidate the wonderful geognostic 

 construction of these islands. On San Michael is situated the enormous 

 Caldeira das sete Cidades. which was formed in the year 1444, almost 

 under Cabral's eyes, a crater of elevation which encloses two lakes, the 

 Lagoa grande and the Lagoa azul, at a height of 876 feet. The Cal- 

 deira de Corvo, of which the dry part of the bottom is 1279 feet high, 

 is almost of the same circumference. Nearly three times this height are 

 the craters of elevation of Fayal and Terceira. To the same kind of 

 eruptive phenomena belong the innumerable but ephemeral platforms 

 which were visible only by day, in 1691, in the sea around the island 

 of San George, and in 1757 around San Michael. The periodical inflation 

 of the sea-bottom, scarcely four miles to the west of the Caldeira das 

 sete Cidades, producing a larger and somewhat more permanent island 

 (Sabrina), has already been mentioned (Cosmos, vol. i, p. 241). 

 Upon the crater of elevation of Astrui, in the Phlegnean plains, 

 and the trachytic mass driven up in its centre, as an unopened bell- 

 shaped hill, see Leopold von Buch, in Poggend. Annalen. Bd. xxxvii, 

 s. 171 and 182. A fine crater of elevation is that of Rocca Monfina. 

 measured and figured in Abich's Geolog. Beobacht. tiber die vulkan, 

 Erschein. in Unter-und Mittd Italien, 1841, Bd. i, s. 113, Taf. ii. 



