CRATERS OP ELEVATION. 217 



say, a crater-like, round or oval depression, bounded by a 

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

 in places ; sometimes (when the frame-work of a permanent 

 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 bot- 

 tom 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 

 exceed the margins of the crater of the cone of elevation. 

 The signs of the first eruption,, the old frame-work, are not, 

 however, always retained. The high wall of rock which sur- 

 rounds the inner circular wall (the crater of elevation) is not 

 recognizable, even in scattered detritus, on many of the larg- 

 est and most active volcanoes. 



It is a great merit of modern times not only to have more 

 accurately investigated the peculiar conditions of the forma- 

 tion of volcanoes by a careful comparison of those which are 

 widely separated from each other, but also to have intro- 

 duced more definite expressions into language, by which the 

 heterogeneous features of the general outline, as well as the 

 manifestations of volcanic activity, are distinguished. If we 



of Mont d'Or, the theory of craters of elevation and their essential dif- 

 ference from the true volcanoes was already expressed. An instruct- 

 ive counterpart to the three craters of elevation of the Canary Islands 

 (on Gran Canaria, Teneriffe, and Palma) is furnished by the Azores. 

 The admirable maps of Captain Vidal, for the publication of which we 

 are indebted to the English Admiralty, elucidate the wonderful geog- 

 nostic 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 incloses two 

 lakes, the Lagoa grande and the Lagoa azul, at a height of 876 feet. 

 The Caldeira 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 ephem- 

 eral 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 some- 

 what more permanent island (Sabrina), has already been mentioned 

 (Cosmos, vol. i., p. 242). Upon the crater of elevation of Astruni, in 

 the Phlegrsean 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. 

 iiber die Vulkan. jErschein. in'Unter-und Mittel Italien, 1841, bd. i., s. 

 113, taf. ii. 



VOL. V. K 



