1856.] SCROPE CRATERS AND LAVAS. 335 



The result of the irregular alternation of these paroxysmal explosions 

 and subsequent gradual expulsions of new matter is the appearance, 

 so common in volcanic mountains, of a minor and central cone with 

 its crater, rising within the circumference of some larger crater of 

 earher date, or in its immediate vicinity. The walls of the latter 

 crater are of course often broken down on one or more sides (gene- 

 rally on the line of the original fissure) ; — perhaps reduced to a mere 

 segment of its original circuit, by the combined operation of volcanic 

 convulsions and aqueous erosions. Whoever will take the trouble 

 to examine carefully an accurate map, on a sufficiently large scale, of 

 almost any volcauic district (such, for example, as Vesuvius and the 

 Phlegrsean Fields, Etna and the Lipari Isles, the Roman Territory, 

 the Grecian Archipelago, Madeira, Teneriffe, the Azores, Bourbon, 

 St. Helena, Barren Island, the Leeward Isles, &c.), will see numerous 

 unquestionable examples of this law by which crater is formed within 

 crater, and new cones upon the ruins of old ones. 



History of Vesuvius. — At the risk of repetition, I must be per- 

 mitted to illustrate this law by the trite, but instructive, example of 

 Vesuvius, — which only comes so often before us because from its 

 proximity to Naples it has been open to more constant and accurate 

 observations than any other known volcanic mountain. What, in 

 brief, is the history of this volcano during the last century? Pre- 

 cisely one hundred years ago, in the year 1756, Vesuvius possessed 

 no less than three cones and craters, one within the other, like a 

 nest of boxes, besides the great encircling crater and cone of Somma 

 (fig. 1). Sir W. Hamilton gives us a drawing of its appearance in 

 this state. 



Fig. 1. — Outline -sketch of Vesuvius as it existed in 1756. 

 (After Sir W. Hamilton.) 



a, Somma. 



By the beginning of the year 1767, the continuance of moderate 

 eruptions had obliterated the inmost cone and increased the inter- 

 mediate one, until it very nearly filled the principal crater (fig. 2, a, b). 

 An eruption in October of that year, 1767, completed the process, 

 and re-formed the single cone into one continuous slope all round 



