TRUE VOLCANOES. 



419 



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

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



ence of parasitic craters on the circumvallation itself. The result of 

 all the careful observations of Julius Schmidt, who is occupied with 

 the continuation and completion of Lohrmann's Topography of the 

 Moon, establishes " that no single central mountain attains the height 

 of the wall of its crater, but that in all cases it probably even lies, togeth- 

 er with its summit, considerably below that surface of the moon from 

 which the crater is erupted." While the cone of ashes in the crater of 

 Vesuvius, which rose on the 22d of October, 1822, according to Brios- 

 chi's trigonometrical measurement, exceeds in height the Punta del 

 Palo, the highest edge of the crater on the north (618 toises above the 

 sea), by about 30 feet, and was visible at Naples, many of the central 

 mountains of the moon, measured by Madler and the Olmiitz astrono- 

 mer, lie fully 6400 feet lower than the mean margin of circumvalla- 

 tion, nay, even 100 toises below what may be taken as the mean sur- 

 face level of that part of the moon to which they respectively belong 

 (Madler, in Schumacher' s Jahrhuch fur 1841, p. 272 and 274; and Jul. 

 Schmidt, Der Mond, 1856, s. 62). In general the central mountains, 

 or central mountain masses of the moon, have several summits, as in 

 Theophilus, Petavius, and Bulliald. In Copernicus there are six cen- 

 tral mountains, and Alphonsus alone exhibits a true, central, sharp- 

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

 Phlegrsean 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 pernMwient 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." (Poggendorflf' s Annakn, 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 southwestern wall 

 of the crater. The summits form two parallel ridges, covered with 

 thick bushes (Julius Schmidt, Eruption des Vesuvs, s. 147, and Der 3Iond, 

 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 tumor or cupola, and is crowned besides with a central 

 mountairw The convexity here is a permanent form. In our terres- 

 trial volcanoes the flooring of the crater is only temporarily raised by 

 the force of internal vapors, sometimes almost to the height of the mar- 

 gin of the crater, but as soon as the vapors force their way through the 

 floor sinks down again.- The largest diameters of craters on the earth 

 are the Caldeira;de Pogo, according to Charles Deville 4100 toises 

 (4*32 geographic^ miles), and the Caldeira de Palma, according to 

 Leop. von Buch 3100 toises ; Avhile, on the moon, Theophilus is 50,000 

 toises, and Tycho 45,000 toises, or respectively 52 and 45 geographical 

 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 cen- 



