TRUE VOLCANOES. 449 



If, as I would fain hope, what I here propound regarding 

 the classification of the volcanic rocks ; or, to speak more 

 precisely, on the arrangement of the trachytes according to 

 their composition, excites any particular interest, the merit 

 of this classification is entirely due to my friend and Sibe- 

 rian fellow-traveller, Gustav Rose. His accurate observa- 

 tion of nature, and the happy combination which he possesses 



274, and Jul. Schmidt; Der Mond, 1856, s. 62). In general, the central 

 mountains, or central mountain-masses of the moon have several sum- 

 mits, as in Theophilus, Petavius and Bulliald. In Copernicus there are 

 6 central mountains, and Alphonsus alcyie exhibits a true central sharp 

 pointed peak. This state of things recalls to mind the Astroni in the 

 Phlegraean fields, on whose dome-formed central masses Leopold von 

 Buch justly lays much stress. " These masses," he says, " like those 

 in the centre of the annular mountains of the moon, did not break forth. 

 There existed no permanent connection with the interior, no volcano, 

 but they rather appeared like models of the great trachytic unopened 

 domes so abundantly dispersed over the earth's crust, such as the Puy 

 de Dome and Chimborazo." (Poggendorffs Annalen, Bd. xxxvii, 1836, 

 p. 183.) The circumvallation of the Astroni is of an elliptic form, closed 

 all round, and rises in no part higher than 830 feet above the level of 

 the sea. The tops of the central summits lie more than 660 feet lower 

 than the maximum of the south-western wall of the crater. The sum- 

 mits form two parallel ridges, covered with thick bushes (Julius Schmidt, 

 Eruption des Vesuvs. s. 147, and Der Mond, s. 70 and 103). One of 

 the most remarkable objects, however, on the whole surface of the moon 

 is the annular mountain-range of Petavius, in which the whole internal 

 floor of the crater expands convexly in the form of a tumour or cupola, 

 and is crowned besides with a central mountain. The convexity here 

 is a permanent form. In our terrestrial volcanoes the flooring of the 

 crater is only temporarily raised by the force of internal vapours some- 

 times almost to the height of the margin of the crater, but as soon as 

 the vapours force their way through, the floor sinks down again. The 

 largest diameters of craters on the earth are, the Caldeira de Fogo, ac- 

 cording to Charles Deville 4100 toises (4 '32 geogi-aphical miles) and the 

 Caldeira de Palma, according to Leop. v. Buch 3100 toises, while on 

 the moon, Theophilus is 50,000 toises, and Tycho 45,000 toises, or 

 respectively, 52 and 45 geogr. miles in diameter. Parasitic craters, 

 erupted from a marginal wall of the great crater, are of very frequent 

 occurrence on the moon. The base of these parasitic craters is usually 

 empty, as on the great rent margin of the Maurolycus ; sometimes, but 

 more rarely, a smaller central mountain, perhaps a cone of eruption, is 

 seen in them, as in Logomontauus. In a beautiful sketch of the 

 crater-system of Etna, which my friend Christian Peters the Astro- 

 nomer (now in Albany, North-America) sent me from Flensburg in 

 August 1854, the parasitic marginal crater, called the Pozzo di Fuoco, 

 which was formed in January 1833, on the east-south-east side, and 

 which had several violent eruptions of lava, is distinctly recognisable, 

 VOL. V. 2 G 



