Mr. PouLETT ScROPE OH the Volcanic District of Naples. 343 



nature. Elsewhere it passes into compact and slaty clinkstone ; and thence 

 again into a very large-grained and saccharine trachyte^ of a dead-leaf colourj 

 composed of glassy felspar. It rests on an outcropping bed of tertiary clay^ 

 full of shells, (of the subapennine formation,) which is extracted for making 

 pottery, from excavations opened under the trachyte*. 



The Lava del Arso, produced by the eruption of 1302, has been fully de- 

 scribed by preceding writers. The point from which it issued in the side of 

 the mountain is marked by a very small crater surrounded by a circular ridge 

 composed of loose black and red scoriae, containing numerous felspar crystals, 

 spongy, light, and approaching (as do all the scoriae of the graystone lavas) 

 to pumice. Near the summit of the Monte Epomeo, the highest eminence 

 of this conoidal island, are distinct traces of two very large craters. That 

 facing the south seems to have been of very large dimensions, and the prin- 

 cipal vent of the explosions of the volcano. 



Vesuvius and Ischia stand at the two extremities of the district I have un- 

 dertaken to describe. The interval between them presents a considerable num- 

 ber of ridgy and more or less insulated hills, of very inferior size and height^ 

 rising from the sea-shore, or from the general level of the plain of the Terra 

 di Lavoro. They are composed, without exception, of trachytic conglomerate 

 (or tufa), of which the greater part is consolidated into a rock resembhng 

 trass ; and, though exceedingly light and porous, of sufficient coherence to be 

 in general use at Naples as building-stone. This rock is stratified, or rather, 

 like most alluvial conglomerates, zoned by alternating layers of different-sized 

 fragments. Above it, is generally seen a series of similar and conformable 

 layers of incoherent tufa or lapillo ; of which also the flat ground between and 

 around the hills consists to the extreme depth disclosed by ravines or other 

 excavations. 



Many of these hills have the ordinary figure of simple volcanic cones, with 

 a central depression or crater. Such are the Monte Barbaro, the Solfatara, 

 Astroni, Capomazza, the hills surrounding the Lago d'Averno, the Monte 

 Nuovo, the western point of Procida, the Capo di Miseno, and the little island 

 of Nisida. Others have the form of steep and narrow ridges, only partially 

 embracing the hollows which appear to have been their craters; whilst some 



* In the autumn of the last year, 1827, and since this paper was read, an earthquake has de- 

 stroyed the village of Casainicciola, which is built in part over these excavations ; and indeed some 

 accounts attribute the catastrophe solely to the falling in of the ill-supported roofs of these clay- 

 pits, but it was probably a slight local earthquake that occasioned their subsidence. Vesuvius 

 having been wholly quiescent since its ebullition in 1822, it would be nothing surprising if the 

 eruptive force of this volcanic focus were to find a new vent for a time in Ischia. 



