244 Proceedings of the British Association, 



taking place by steps and indicated by the position of the dykes, 

 which gradually partake more and more of the mineral character 

 of those connected with the present craters. He next called atten- 

 tion to the circumstance that the central crater was situated on 

 one side of a great dome-shaped elevation, for the formation of which 

 something more than the successive deposition of lava streams 

 had been necessary. The idea that the principal mass of a volcano 

 like Etna might be formed in this way, had arisen from an erroneous 

 estimate of the quantity of liquid matter erupted at any one time, 

 of the manner in which it was spread out, the equality of its distri- 

 bution, and the angle at which it could be consolidated. On the 

 principle advocated by Mr. Lyell, that the greatest effect might 

 be produced by small causes, multiplied indefinitely, it had been 

 inferred that a mountain 10,000 feet high might be formed in a 

 period proportionally longer than one of only 1,000 feet by the same 

 means. In fact, that Etna might be formed by a long-continued 

 series of eruptions such as produced the Monte Rossi. This Prof. 

 Forbes considered quite impossible, for a reason pointed out by 

 M. Elie de Beaumont, who, in describing Mount Etna, had seized 

 upon many of its leading features, and given an explanation of its 

 structure, fully borne out by Baron Waltershausen's elaborate 

 survey. The sections in the Val del Bove exhibited vast numbers of 

 alternating strata of ancient lavas, separated by thin layers of ashes 

 or slags, more compact and more evenly distributed than those of 

 modern date, having similar angles of inclination. — Mr. Hopkins 

 remarked that there were three points of great importance to be con- 

 sidered: 1. The actual constitution of the great mass of Mount 

 Etna, considered by M. de Beaumont a cone of elevation, and not 

 of successive eruption; his argument being similar to that which 

 geologists apply to ordinary strata, when they are found at an angle 

 greater than that at which they could have been originally deposited 

 by water. The approximate thickness of the beds of lava being the 

 same as usual, they must have been brought into their inclined 

 position by subsequent elevation. 2. The lines of contemporaneous 

 dykes diverging from accurate centres ; an arrangement which might 

 have been inferred a priori from mechanical considerations. 3. The 

 circumstance that there are distinct systems of these diverging 



