522 PROCEEDINGS OF THE aEOLOGICAL SOCIETY. [Feb. 2, 



scarcely fail to create such a cone as we now see around the orifice 

 of projection — which is not nearly so big as the Monte Rosso and 

 many other acknowledged cones of eruption. 



The unwillingness of M. Dufrenoy to admit that Monte Nuovo 

 was the product of eruptions is by himself grounded on the perfect 

 similarity of the tuff-beds of which it is composed to those which 

 are so generally spread over the Phlegraean fields, and compose the 

 older cones and craters around. It was clearly seen by him, as well 

 as by all the other upheavalists (who are for once agreed on this 

 point), that, if the upheaval- theory is renounced in regard to the 

 former (the Monte Nuovo), it must be equally given up as respects 

 the other hills, including even Somma itself, round which these 

 tuffs mantle conformably. In fact Monte Nuovo is evidently the 

 key to the origin and mode of formation of the neighbouring cones 

 and craters ; and it was therefore necessary at all risks to make it out 

 an upheaval cone. Consequently the bold assertion is advanced by 

 all of them, that Monte ISTuovo was not a new hill in 1538 — that it 

 was not formed at that time, nor by the eruption to which all con- 

 temporary observers attribute its production ! 



I may stop a moment here to say that the similarity of the tuff- 

 beds of which Monte Nuovo is composed to those of the surrounding 

 more ancient crater-hiUs, is, of course, owing to the eruption tha1>«. 

 formed it having been, as in their case likewise, subaqueous, or on 

 the shallow margin of the sea, and the matter thrown up chiefly 

 pumice or felspathic scoriae, which, triturated into ashes by repeated 

 ejection and mixed by agitation with the sea- water into a kind of 

 mud or mortar, hardens into a tough tufa*. The later explosions of 

 the eruption being probably subaerial, or not so much mixed up with 

 water, the superficial beds are found to consist of incoherent tuff, 

 lapilli, &c., remaining just as they fell from the air. And this is the 

 general composition and arrangement of all the tuff-hills of the district 

 of the Phlegraean fields f. An exact parallel in every particular to 

 these tuff- craters seems, from the statement of Mr. Darwin, to occur 

 in the Galapagos Isles near Banks's Cove. He describes one 500 

 feet in depth and f of a mile in diameter, — ^the lower beds being 



middle of the bottom of which the stones that had fallen there were boiling up 

 just as in a great caldron of water that boils on the fire." 



* In the latter days of the Vesuvian eruption of 1822, the fine ashes thrown 

 out by the volcano, and which, mixed with rain-water into mud, were washed down 

 the slopes of the mountain, formed a crust of indurated tuff so compact and hard 

 that it required a pickaxe to break it. Some of its beds were pisolitic, the drops 

 of rain having aggregated the fine ashes into globular concretions. It was an 

 alluvium of this character (mud-lava, lava di fango) which overwhelmed Hercu- 

 laneum, while Pompeii, lying beyond the base of the mountain, was buried imder 

 the loose ejecta of the same eruption, falling upon it from the air. The trass of 

 the Rhine volcanos, and the Moya of the Peruvian volcanos are the result of a 

 similar admixture of water and felspathic ash. In these latter cases, however, 

 the contained infusoria or fish proved the mud-flood to have been produced by 

 the debacle of crater-lakes. The mud-eruption which in great part formed Monte 

 Nuovo is weU described by one of the eye-witnesses of the phenomenon in Sir 

 W. Hamilton's book already quoted. 



t This sviperficial bed of incoherent tufi" may be seen on the cone of Miseno. 

 See fig. 7 above. 



