At the western extremity of the Somma, rises an elongated tufaceous eminence, named Monte Can taroni, on which at an elevation of 1971 feet, stands the well known hermitage of St. Salvador, and which is said to have been thrown up by the erup tion of A.D. 79, when the whole features of the volcano seem to have been changed, and when the present position of the cone probably firrit became the point of emission. This spot is protected from the effects of streams of lava which often pass near it, by a valley on each side, which serve to guide the torrent in different directions; one of these is called the Fossa di Paraonte, the other Fossa Grande; Fossa being the name here for these deep and narrow ravines with which Vesuvius abounds; the latter of these is interesting from the vast abun dance of simple materials which it affords.
The cone, or Vesuvius properly so called, next demands attention. Its height is liable to conside rable variation from the explosive effects of erup tions which have been known to carry off at once 800 feet of perpendicular height of the summit. The Abbe Nollet in 1749 found the height above the sea to be 3120 French feet, while Della Torre in 1752 gave from his very imperfect measurements with the barometer only 1677 feet, a singular proof of the enormous errors which at that period might be committed without immediate detection. The first good observation we have is that of Saussure, making the height 3659 French feet, which is very interesting, by showing, as Humboldt has remarked, that this level is more constant than we are apt to imagine. Shuckburgh found the edge of the crater from which the lava of 1776 flowed to be 3692 French feet = 3935 English. The point which Saussure measured was on the N.W. side of the crater, and from a post which was fixed upon it was named La Rocca del Palo. In 1805 Gay Lussac found this point 3757 English feet, and Lord Minto in 1821, 3963, and at that time, forty-nine years from its first measurement, it was carried away by the great eruption. To methodize the results with as much accuracy as the nature of the operations in general warrants, we give the height of the Rocca del Palo by different observers, in toises merely (= 6.3947 English feet) from a late work of Humboldt's.* There is an appearance of greater elevation dur ing the latter part of this period, which Humboldt thinks may amount to 12 toises, to be considered a proof of gradual internal elevation.
The proportion of the cone of ashes to the total height is in Vesuvius nearly, whereas in the vol cano of Pichincha, it is only and in the Peak of Teneriffe so low as The mean slopes of volca nic cones are, according to Humboldt, from 32,, to 42,,; the mean slope of Vesuvius between the sum mit and the Atrio del Cavallo, is probably equal to the highest of these values. The base of the cone is about 2480 feet above the sea.
The crater by which the cone of ashes is trun cated, has varied in character and dimension very much, according to the state of the volcano. Sub sequently to the eruption of 1822, when so vast a portion of summit was carried away, the crater has been, according to general computation, no less than three miles and one-third in circumference, and 1500 feet deep; being probably the largest in existence. This confirms the view given under
the article PHYSICAL GEOGRAPHY, (Vol. XVI.), that, generally speaking, the size of craters is in versely as the height of the mountains to which they belong; even the case of Pichincha, which has a crater a mile in diameter, there quoted as an ex ception, we now see outdone in the comparatively diminutive example of Vesuvius. The American volcano is no less than four times as high as that of the Bay of Naples, or near 16,000 feet; Mount Etna, which has an elevation of 11,000 feet, had in 1769 a crater only 2.1 miles in circumference, and that of the Peak of Teneriffe, which has a height of above 12,000 feet, is only 300 feet long, 200 broad, and 100 deep.
The edge of the crater is narrow and precipitous, sloping internally and externally so fast, as to leave but a small ridge to walk upon. As to height. it is very irregular, the northern point being 500 feet higher than the southern. The depth of the crater below the lowest edge, is probably little less than 1500 feet, from which some idea of the true size of this vast chasm may be formed. The southern side presents so moderate a slope inwards, that without any peril, the traveller may descend till within about 500 feet of the bottom, where he is stopped by a precipitous crag; in the opposite direction it is so steep, that we cannot descend above a few paces. Various, indeed, are the modifications which the crater has assumed under different stages of volca nic energy. Previously to the first recorded erup tion of A.D. 72, the summit exhibited, according to Strabo, a level plain, interspersed with rocks and caverns, which were rightly interpreted, as bear ing marks of previous imflammation. Before the great eruption of 1631 took place, which succeeded to a long period of repose, the crater exhibited the most deceptive marks of quiescence; it was then 5000 paces round, and 1000 deep; in the bottom was a plane where cattle grazed, and the banks were clothed with abundant forests, in which wild boars took shelter, and afforded sport to the lovers of the chase. The sloping path which led to the bottom is said to have been three miles long. Three small lakes existed there, of which, according to the re port of contemporary writers, one was warm, an other salt, and the third bitter. It appears from the curious old representation of the mountain at this period, copied into Mecatti's account of Vesu vius, that the Monte Somma was covered with trees to the top, as was also the base of Vesuvius. In 1660 the crater was so shallow that it was easy to descend to the bottom, where a small cone was raised by the immediate action of the volcano. In 1755, the bottom was only 23 French feet below the edge, and from the centre rose a parasitical cone 80 or 90 feet in height, with its own proper crater. How different these conditions were from the pre sent one, the dimensions already given will show.