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



ridges run lengthways to a considerable distance in nearly a straight line, and 

 offer few external indications of their volcanic nature. 



The uniform internal structure and composition, however, of these hills» 

 which is here and there observable in ravines, quarries, or abrupt cliffs under- 

 mined by the breach of the sea, proves them all to have been produced by the 

 same process; namely, the accumulation of fragmentary trachyte and pumice 

 projected from one or more volcanic apertures. Their component strata are 

 invariably mantle-shaped, i. e. dip both ways from the axis of the ridge, a cha- 

 racteristic feature of all volcanic cones, dependent on their peculiar mode of 

 formation, and which may be employed as a very useful and accurate guide 

 towards fixing the localities of extinct vents of eruption. The northern escarp- 

 ment of the Capo di Miseno exhibits this structure very satisfactorily, as like- 

 wise does the sea-ward bluff of the isle of Nisida, and the segment of a crater 

 in the bay of Baiee near the village of Bacoli. 



The only one of these hills of which the date is known is the Monte Nuovo, 

 thrown up in the year 1538 ; and this differs in no material points from the 

 others, except that its constituent fragments have rather less of felspar in their 

 mineral composition. Its scoriae are indeed mostly filamentous, passing into pu- 

 mice, but heavier and of a darker colour than the ordinary type of this mineral; 

 and the fragments of solid lava, of which a very large quantity is scattered over 

 the outer slopes of the hill, approach to clinkstone in appearance, are schis- 

 tose, and often (which is remarkable,) veined with laminee of pitchstone. In- 

 deed many pieces are met with that contain passages from an earthy clink- 

 stone to pitchstone, and thence into pumice. In these specimens it appears 

 evident that the clinkstone has been partially changed by fusion into pitch- 

 stone, and this by the sudden development of aeriform fluids into pumice. 

 These different modifications of the same substance alternate with one another 

 in repeated zones within the same block, and are such as would probably be 

 produced by exposing to artificial heat a mass of this rock, in which some 

 laminae might be, from their peculiar mineral character or fineness of grain, 

 inclined to enter into fusion more readily than others. The more or less rapid 

 cooling of the substance has, here, at least, had no effect in modifying its 

 texture. 



Next to the Monte Nuovo, the cone which may be judged to have been 

 most recently produced is that of the Solfatara, whose w ell known phenomena 

 attest that a mass of lava still exists beneath it at an intense temperature, send- 

 ing forth through superficial fissures torrents of aqueous vapour impregnated 

 with sulphuretted hydrogen. This gas, on coming in contact with the atmo- 

 sphere, combines with its oxygen into sulphuric acid, which produces a series 



