Mr. PouLETT ScROPE ou the Volcanic District of Naples. 345 



of remarkable alterations in both the tufa and compact trachyte on which it is 

 deposited : the former, from the greater facility with which it is penetrated by 

 the acid, being affected to a great depth ; the latter only superficially. Sul- 

 phates of alumine, iron, lime, magnesia, and soda, result from the decomposition 

 of these rocks by the sulphuric acid ; and their silex left nearly pure, in the 

 form of a white earthy powder, is carried down by the rains, and deposited in 

 thin beds in the lower levels of the crater. The very porous nature of these 

 beds no doubt occasions the loud reverberation produced when they are 

 struck sharply, which is vulgarly attributed to the existence of a vast vaulted 

 cavity beneath. They consist of a very fine white clay, indurated by heat^ 

 and penetrated by numerous vesicular cavities, occasioned by the generation 

 of some gaseous fluid during its consolidation : in all probability by the rare- 

 faction of the air or water it contained, while exposed in the soft state of mud 

 to the influence of the heat transmitted from the volcanic focus below. This 

 hardened clay is divided into the finest laminae, much thinner than paper, has 

 the appearance of tripoli, and might probably be applied to the same uses. 



I have in another place* dwelt sufficiently at length on the futility of sup- 

 posing any cavity, vaulted or not, to exist under a volcano. The very circum- 

 stance of the violent protrusion of solid and liquid matters, which constitute 

 the characteristic phaenomena of an eruption, sufficiently proves that, so far 

 from there being at that time any empty space beneath it, there is too little 

 room for the mass of heated matter already confined there in a state of enor- 

 mous tension by the weight and tenacity of the overlying rocks, and which in 

 consequence at length uplifts or bursts through them with a sudden and 

 violent intumescence. After every eruption, the residuum of ebullient lava 

 left in the volcanic focus, having been by the generation of vapour through- 

 out its substance cooled down below the temperature of the surrounding 

 mass, begins immediately to receive rather than to part with caloric, and con- 

 sequently to augment not to diminish its tension. No cavity can therefore 

 be formed during this time. 



The Solfatara has produced a massive current of trachyte, which stretches 

 with a gradual slope in the form of a rocky promontory, into the sea. The 

 upper extremity (called Monte Olibano) impends over the crater of the Solfa- 

 tara in a precipitous cliff. It is hardly necessary to say that the crater must 

 of course have been hollowed out by aeriform explosions subsequently to the 

 flowing of this lava, which is a decided trachyte, but varies extremely in grain, 

 texture, and even in mineral character. The upper part of the current con- 

 tains much less augite than the lower, and its felspar crystals are larger and 



* Considerations on Volcanos, p. 267-8. 



VOL. II. — SECOND SERIES. 2 Z 



