338 Mr. PouLETT Scrope on the Volcanic District of Naples. 



ranges of elevated strata,, that not only is there along the western coast of Italy 

 this correspondence between two decided trains of volcanic vents, and the two 

 parallel ranges of the Sabine mountains and the Apennines ; but also, where a 

 massive embranchment breaks off from the latter at nearly a right angle, 

 forming the promontory which bounds the bay of Naples to the S.E. and the 

 almost continuous island of Capri, a corresponding line of volcanic vents (viz. 

 that connecting Vesuvius with Ischia,) is found to take off from the principal 

 line, at no great distance, and at the same angle, so as consequently to run 

 parallel with the transverse elevated range. 



If we call to mind the general tendency of the rocks composing the secon- 

 dary strata to yield to any disruptive force along fracture-lines at right angles, 

 or nearly so, one to another, it will appear highly credible that this parallelism 

 in two directions of the ranges of elevation and eruption, has been influenced, 

 here as elsewhere, by that general, though at first sight insignificant, cause. 



Within the space above limited, the volcanic force has at times developed 

 itself in single eruptions, from a considerable number of irregularly scattered 

 points ; but has shown a decided preference to two principal and habitual 

 vents, Vesuvius or Somma, and Ischia. These by the accumulated products 

 of repeated eruptions have swelled into bulky excrescences, Avhich exhibit, 

 though with some difference, the characteristic features of ordinary volcanic 

 mountains. Of the two, Ischia has been decidedly the most prolific source of 

 erupted matter ; since that portion of it which is seen above the water-level 

 surpasses Somma considerably in bulk, and a great part of its mass must be 

 supposed hidden by the sea. These two mountains also differ materially in 

 figure and constitution. This is chiefly attributable to the different mineral 

 character of the lavas which each has emitted. 



Somma is an exceedingly regular volcanic mountain, though on a small 

 scale ; its lavas, (at least all that are disclosed to view) are of leucitic basalt ; 

 and although its conglomerates are not entirely basaltic, some containing a 

 large proportion of pumice and feldspathose lava-blocks, yet these are un- 

 accompanied by any masses of trachyte in situ, and may be supposed to be 

 the scattered fragments of some of the inferior beds belonging to an earlier 

 eruj)tive aera, at which the volcano may have been productive of trachyte, and 

 broken up by some of the later paroxysmal explosions. 



However this may be, all the visible lavas and the greater part of the con- 

 glomerates of Somma are basaltic ; and owing to the great Jluiditj/ of lavas of 

 this mineral character, they have, when protruded from the common vent, 

 taken their course in spreading sheets down the outer slope of the mountain ; 

 while the scoriae and fragmentary matters, projected at the same time into the 



