Mr. PouLETT ScROPE oti the Volcanic District of Naples. 339 



air, were spread pretty evenly over them : so that the result of successive 

 eruptions of this kind has been the formation of a very regularly conical 

 mountain, with a gradually diminishing slope on all sides, from the central 

 heights to the plain around ; exhibiting in the ravines that furrow its sides, 

 as well as in the abrupt sections afforded by the walls of the great crater, 

 its composition of repeatedly alternating beds of basalt and basaltic conglome- 

 rates, more or less irregular in thickness, but dipping uniformly on all sides 

 away from the vent, with an inclination corresponding exactly to the external 

 slopes of the mountain. 



The eruptions of Somma seem very rarely to have taken place from any 

 other than the central vent. The small cone on which the Camaldoli della 

 Torre is built, and those still smaller ones immediately above Torre del Greco, 

 which were thrown up by the eruption of 1794, are the only indications of 

 explosions having burst forth from the side of the mountain. The vast number 

 of vertical basaltic dykes which intersect the horizontal beds observable in the 

 broken cliffs of the old crater (Atrio del Cavallo) bear witness, however, that 

 the lava was not so frequently elevated to the summit of the mountain with- 

 out occasioning numberless cracks and rents in its inmost structure ; which, 

 though occupied immediately by the liquid lava, seem never but in the in- 

 stances above-mentioned, to have opened so far as to allow the explosions to 

 find their way out at the side of the cone. There is great reason to conclude 

 that the old crater of Somma, whose steep walls now half encircle the cone of 

 Vesuvius, was formed by the celebrated paroxysm of the year 79, which occa- 

 sioned the death of the elder Pliny. The enormous quantity of fragmentary 

 matter projected into the air by the explosions of that eruption, which not 

 only buried Herculaneum, Pompeii, and Stabiee, under a bed from thirty to 

 one hundred feet in thickness, but seems to have covered Naples itself to a 

 depth of about ten feet, (as appears in the excavations behind the Studii 

 Reali*,) and filled the atmosphere to such a degree as to produce complete 

 darkness, according to Pliny the younger, for nearly three days, must have 

 left a proportinate cavity in the mountain. The very analogous, though minor 

 explosions of 18£2, which completely gutted the cone of Vesuvius, leaving a 

 crater a mile in diameter, and at least eight hundred feet in depth, did not 

 eject a sufficient body of ashes and fragments to cover the base of the moun- 

 tain to more than the average depth of a foot; nor at Naples, though the 



* In this spot, above some made ground containing both Greek and Roman tombs, there is 

 seen a deep deposit of lapillo, exactly similar to that under which Pompeii is buried, and which, 

 from the way in which its strata lie, appears to have been formed at one epoch, most probably 

 that of the eruption of Titus. See PI. XXXIV. fig. 1. 



2 Y 2 



