Mr, Lobleys Lecture on Vesuvius. 823 



Vesuvius, Monte Nuovo, on the shore of the Bay of Naples, was 

 thrown up. This event took place in 1538, and the phenomenon 

 was of so remarkable a character, that a hill 440 feet high, and more 

 than a mile and a half in circumference, was formed by the accumu- 

 lation of erupted matter in three days ! During the eruption of 

 1793-4, which lasted upwards of a year, the stream of lava emitted 

 was not even arrested at the sea line, but actually flowed into the 

 sea to a distance of 362 feet, forming a promontory. It was calcu- 

 lated by Breislack that this stream contained upwards of 46,000,000 

 cubic feet of lava. The eruption of 1822 was by far the greatest 

 outbreak which has occurred during the present century. The 

 modification of the shape of the volcano caused by this eruption was 

 very considerable, since no less than 800 feet of the summit of the 

 cone was blown away, and an immense crater produced, which 

 penetrated to a depth of not less than a thousand feet into the heart 

 of the mountain. 



Considering Vesuvius geologically, we find the lower portion of 

 the volcano as far up as the Crocelle, or the ridge on which stands 

 the Observatory, to be of one age, Monte Somma of another, and the 

 great cone more modern than either of the other two portions of the 

 mountain. The formation of the great cone of Vesuvius, as we have 

 seen, dates from a.d. 79, before which time it had no existence. This 

 very interesting part of the volcano is simply the accumulation of 

 the ejectamenta thrown out during the various eruptions of modern 

 times, with interbedded layers of lava, and is consequently made up 

 of a series of beds, or coats, having a quaquaversal dip from a central 

 axis, which is the vent of the volcano, and the mouth of which forms 

 the crater or cup-shaped hollow at the apex of the cone. These beds 

 were found to have a dip of from 26*^ to 30°, but the exterior slope 

 of the cone has an inclination of 40°. The great eruption of 1822 

 left so wide and deep a crater that the internal structure of the cone 

 was exposed, and it was then found that dykes of compact basalt 

 crossed the beds of lava and scoriee, and rose more or less vertically 

 to various heights. These walls of hard rock have doubtless been 

 produced by the filling up of old lava channels and cracks in the 

 cone with liquid lava which has afterwards solidified, and they act 

 as so many ribs, greatly strengthening the structure of the cone. The 

 crater varies in size and shape with almost every eruption, as the 

 greater paroxysmal eruptions tear away the sides of the mouth of the 

 volcano, and leave a very large and deep abyss. This is afterwards 

 rapidly filled up by the accumulation of the lava, ashes, cinders, and 

 lapilli discharged during the minor eruptions which follow, until we 

 find a new cone growing up above the former summit of the moun- 

 tain, and this has at its apex only a small crater, as is the case at the 

 present time. The central vent is then almost choked, and so it 

 remains until another paroxysmal eruption occurs, when the new 

 cone is blown away, and a great crater again produced. 



Monte Somma we find to be a semicircular ridge of about two 

 miles in length, with the face opposite to the cone, forming an almost 

 perpendicular escarpment. This, being a portion of the enclosing 



