126 Sir Charles Lyell [April 15, 



inferred, when once the law above laid down respecting the consolida- 

 tion of melted matter had been accepted, that every mountain contain- 

 ing such inclined and compact layers, must owe its conical form almost 

 exclusively to the development of mechanical force exerted at the close 

 of the volcanic operations, or after all the alternating lavas and scoriae 

 were heaped up. The hypothesis of a sudden and violent movement 

 was perhaps the more readily embraced, because it relieved its 

 advocates from the necessity of making unlimited drafts on past time, 

 thousands of centuries being required if lofty cones, like Mount Etna, 

 are to be built up by successive eruptions of ordinary intensity. The 

 magnitude also of certain craters or " calderas " (implying, probably, 

 one or more great explosions, followed by aqueous erosion), and the 

 occasional steepness of the dips of certain lavas, beyond that which is 

 found on the flanks of ordinary cones, (many of which might have been 

 assigned to local dislocation,) afforded additional arguments in favour 

 of the new hypothesis. The lecturer then gave a rapid review of the 

 controversy respecting " craters of elevation," stating the objections 

 made to it by English and Continental writers, including the late M. 

 Constant Prevost ; and he went on to observe that the principal object 

 of this discourse was to show that the law laid down by M. E. de 

 Beaumont, and by the late M. Dufrenoy, as governing the cooling 

 and solidification of lava currents, on steep slopes, has no foundation in 

 fact. Signor Scacchi had already, in 1855, seen and described a com- 

 pact stony lava which in that year had flowed down the flanks of 

 Vesuvius from near the margin of the great crater to the base of the 

 cone in the Atrio del Cavallo, having a thickness of from 1^ in the 

 upper to 4t in its lower part, and dipping at angles varying from 

 32° to 38°. The interior of this current was laid open to view by a 

 rare accident, namely, the sinking down in the same year (1855) of a 

 certain portion of the north flank of the cone, whereby one side of the 

 new lava stream was engulfed, and a section of the remainder rendered 

 visible. Although this current had cooled on an average declivity of 

 35°, it was as compact and as free from vesicles as many lavas which have 

 congealed on level ground at the foot of Vesuvius.* 



The first exemplification of a similarly inclined stony lava of known 

 date on Mount Etna, described by the lecturer, and of which a pictorial 

 representation was given, occurs in a ravine called the Cava Grande, 

 near Milo, about 17 miles north of Catania, and 7 from the sea, 

 above the level of which it is elevated about 2000 feet. A branch of 

 the lava-current of 1689 descending from the Val del Bove, cascaded 

 over the right bank of that ravine 220 feet high, and on cooling, formed 

 a tabular mass more than 16 feet in thickness, inclined at an average 

 angle of about 35°, and concealing the face of the precipice for a width 

 of about 400 feet. The internal structure of this new lava has been 

 exposed to view by the falling down and partial removal of its scoria- 



* This section, seen by Signor Scacchi in 1855, was looked for by Sir C. Lyell, 

 in company with Signor Scacchi in 1857, and found to be totally buried and con- 

 cealed by the lavas poured out in the early part of that year. 



