350 J. W. Judd—On Volcanos. 



of a volcano, and may fairly employ it for comparison with others, 

 concerning the formation of which we have neither historical records 

 nor traditions to aid us, and which may possibly indeed have origin- 

 ated prior to the appearance of the human race upon the earth. 



Such an example we have in the great volcano of Eocca Monfina, 

 which presents so many points of analogy with Vesuvius that the 

 geologist will have no difficulty in recognizing the mode of origin of 

 the principal features of the former, though it has long been extinct, 

 and its rocks have suffered greatly from the action of denuding forces. 



The mountain group of Eocca Monfina exhibits a crater-ring of 

 about three miles in internal diameter, that is to say, it is somewhat 

 greater than the similar crater-ring of Somma, which surrounds the 

 modern cone of Vesuvius. The materials which compose these older 

 encircling craters of Somma and Eocca Monfina are almost identical, 

 namely, leucitic basalts and the tuffs derived from them ; but it is 

 clear that while in the former the lavas form a very large proportion 

 of the mass, in the latter they are quite subordinate to the tuffs, of 

 which the volcano is mainly built up. In the centre of each of these 

 old craters rises a more modem volcanic cone, but of very different 

 characters in the two cases. While Vesuvius is composed of lavas 

 and tuffs quite similar in character to those of Somma, the Montagna 

 di Santa-Croce, which has risen in the midst of the old crater of Eocca 

 Monfina, consists of vast hummocky masses of a peculiar rock — a 

 " trachy-dolerite," with much mica. That the crater-ring of Corti- 

 nella (which embraces the mountain produced by later eruptions, in 

 the same manner that Somma does Vesuvius) was formed by 

 similar explosive action to that which we know gave origin to 

 the latter, no one can doubt who observes the exact correspondence 

 in all the characters of the two mountains. The only difference be- 

 tween them is this — that while Somma, after the great paroxysm 

 which destroyed all the higher and central portions of its mass, con- 

 tinued to pour forth those similar leucitic lavas and tuffs by which 

 the modern cone of Vesuvius was gradually built up, Eocca Monfina, 

 by a change not uncommonly witnessed at centres of volcanic out- 

 bursts, began to originate materials of a different composition and 

 mode of behaviour, namely, the more acid lavas of much less perfect 

 liquidity which formed those great bosses in the centre of its crater 

 constituting the mountain-masses of Santa-Croce. 



Proceeding still to the northwards, we find, a little to the south of 

 Eome, a third volcanic group, that of Monte d'Albano, composed of 

 similar leucitic basalts and tuffs to those of Vesuvius and Eocca 

 Monfina. In the centre rises Monte Cavo, which we may justly 

 compare to Vesuvius ; it is a volcanic cone, with a well-marked 

 crater at its summit, upon the floor of which rise the remains of 

 several smaller cones, now weathered clown and grass-gi'own. Monte 

 Cavo, like Vesuvius, is embraced by a great crater-ring, broken away 

 on its western side by the later parasitical eruptions which have origin- 

 ated the craters of Vallariccia, Lago d'Albano, Lago di Nemi, and 

 the craters about Frascati. But while the outer crater-ring of Somma 

 has an internal diameter of only two miles and a half, and that of 



