Geological Society. 387 



At one extremity of the tract in question lies the habitually erup- 

 tive volcano ofSomma; at the other the once active vent of Ischia; 

 the intermediate space is studded with hills, evidently thrown up by 

 numerous eruptions, succeeding one another at distant intervals, 

 and from separate though neighbouring orifices. These are arranged 

 in one general band, which is remarkable from its parallelism to the 

 elevated limestone range forming the opposite side of the Bay of 

 Naples, and separating it from that of Salerno. 



Somma is a very regular volcanic mountain, created by the accu- 

 mulation of repeated streams of basaltic lava and beds of ejected 

 ashes, sand and scoria, round a central and habitual vent. 



The author dissents from the theory of Von Buch, that such 

 mountains were produced by the forcible elevation of horizontal 

 beds round an aperture of eruption ; — though he allows that beds 

 originally inclined, may often suffer a certain degree of elevation, 

 during the shocks occasioned by the forcible protrusion of lavas 

 from below, into the fissures through which they are emitted. 



The great crater of Somma is attributed to the explosions of the 

 " paroxysmal eruption" of A. D. 79 ; and the whole cone of Ve- 

 suvius which occupies the centre of that crater, is stated to have 

 been created by repeated subsequent eruptions. This cone is si- 

 milar in structure to that of Somma, as is seen in the walls of its 

 actual crater, compared with those of the Atrio del Cavallo. 



Ischia is a less regular volcanic mountain; has produced no leu- 

 cite, and none but trachytic, or rather, according to the author's 

 nomenclature, gray-stone lavas, — a class intermediate between tra- 

 chite and basalt, and consisting of felspar and augite. The great 

 mass of the island is composed of the conglomerates belonging to 

 this class of lavas, forming an indurated tufa of a light green colour. 

 There are traces of a vast central crater on the west of the Monte 

 Epomeo. Some of the lavas of Ischia are remarkably brecciated 

 and zoned, — with varieties of grain, texture, and mineral composi- 

 tion. 



The intermediate district between Somma and Ischia, properly 

 called the Campi Phlegrei, including the islands of Procida and Ni. 

 cida, exhibits the traces of between twenty and thirty crateriform 

 basins, many of very large diameter, but in general much degraded, 

 and sometimes almost obliterated, by the erosive action of the sea 

 and of rains on the loose conglomerates of which they are partly 

 composed, and b}' the ejections of later neighbouring eruptions. 

 Ten at least of these cones, with their included craters, are however, 

 very nearly entire ; such are the Monte Nuovo, produced in the 

 year 15.38 ; Capo Mazza, a hill entirely composed of silky pumice 

 and its detritus ; the Monte Gauro, which incloses a deep circular 

 crater a mile in diameter ; Astroni, which is nearly equal to the 

 last in size, and precisely similar in figure ; the basins of the lakes 

 Averno and Agnano ; the island of Nicida ; the southern extremity 

 of the island of Procida ; the Capo di Miseno ; and the Solfatara of 

 Pozzuoli. 



The author disputes the existence of any large vaulted cavity 



under the floor of the last-nientioncd crater ; and attributes the rever- 



3 D 2 beration 



