1859.] SCROPE CONES AliTD CRATEES. 528 



formed of compact tuff, appearing like a subaqueous deposit; the 

 upper of a harsh, friable, hght, and occasionally pisolitic tuff; the 

 beds of each dipping regularly away on all sides from the crater, at 

 an angle of from 25° to 30°. But within the crater are other strata of 

 tuff dipping at a still higher angle inwardly. Mr. Darwin says truly 

 of them, that the appearances could not possibly have resulted from 

 upheaval. Professor Dana describes other tufa isles of the Pacific 

 in similar terms*. 



The tuff- cones and craters of the Phlegraean fields, indeed, only 

 differ from the cones and craters of Lanzarote, Etna, or Central France 

 in being composed of pumiceous (i. e. felspathic) scoriae and ash 

 instead of basaltic scorise and ash, and in being of subaqueous instead 

 of subaerial origin, in consequence of which their materials are 

 more consolidated and stratified, and their form wider, broader, and 

 more openly spread out than the latter subaerial cones. In other 

 respects, of structure, dip, and direction of the component beds, both 

 classes are so exactly alike, as to make it a matter of astonishment 

 that an entirely different and opposite mode of production should be 

 attributed to them by the upheavalistsf ; more especially since, in 

 the case of the Monte Nuovo at least, there is ample contemporary 

 authority from bystanders for the occurrence of those very copious 

 eruptive ejections which these geologists admit to have given birth to 

 the cones of Etna and Erance. 



It is a similar dilemma which has driven the upheavalists into 

 the inconsistencies already referred to respecting the mode of forma- 

 tion of Vesuvius. The perfect analogy of its chief cone, in form, 

 structure, and composition, to the half- encircling cone of Somma, 

 from the centre of whose crater it rises, makes it all but impossible 

 to attribute a totally distinct origin to the two. When, immediately 

 after the great eruption of 1822, I stood on the acute ridge of the 

 prodigious crater that had been drilled through the solid heart of 

 the cone by the gaseous explosions of the previous twenty days, 

 and marked the exact resemblance of its internal cliff-sections to 

 those of the half- encircling crater of Somma, which were within my 

 view at the same moment, I could not doubt that both the inner 

 and outer concentric cones and craters owed their origin to similar 

 developments of eruptive violence. I could as soon hesitate to 

 believe that the separate pieces of a turner's nest of boxes, or the 

 flower-pots that we buy, fitting one into the other, were respectively 

 fashioned by the same process. The irregular beds of lava and con- 

 glomerate traversed by dykes, which visibly composed the cliffs of 

 either crater, dipped on all sides away from the same centre, at the 

 same angle, parallel in either case to the outer slopes of each cone. 



* U.S. Expl. Exp. vol. i. p. 328. 



t M. Rozet, one of the disciples of this school, thus describes all the craters of 

 the Phlegraean fields : " They are not eruptive craters, but simply ' cirques,' opened 

 in the pre-existing horizontal tuff-beds." They are " dislocations en forme de 

 cirque." " C'est une grosse buUe de gaz, qui, apres avoir form6 une ampoule 

 dans les tuffes horizontaux, a fini par les crever." (Rozet, Mem. Soc. Gt6ol. de 

 France, 2 ser. part 1, p. 140.) 



