230 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 19, 



the almost exclusive theatre of eruptions. If such has not heen the 

 case, it is doubtless because the permanence of the site of habitual 

 volcanic vents depends on deep-seated chasms and fissures in the 

 earth's crust extending downwards many leagues, and which cannot 

 be affected by changes of a comparatively superficial nature. 



It is the opinion of M. de Beaumont, that the sheets of compact lava 

 and alternating beds of scoriae, which are now inclined in some of the 

 cliffs encircling theVal delBove at angles of 20° and 27°, were originally 

 so horizontal that the lava emitted from different vents on the platform 

 where they accumulated, flowed with equal freedom in every direction. 

 To this circumstance he attributes the parallelism throughout a wide 

 area, and the compact nature of a vast series of sheets of lava separated 

 by more than 1 00 intercalated beds of pulverulent matter, cinders and 

 angular fragments, such as are commonly cast out of craters during 

 eruptions. The most cogent argument relied upon to compel us to 

 embrace this view of original horizontality, is derived from the alleged 

 fact that many of the dikes, intersecting the perpendicular cliffs alluded 

 to, terminate upwards at different heights, and on reaching particular 

 sheets of lava are there seen to blend, or "articulate" with them. 

 The dikes are therefore imagined to have been feeders, or the channels 

 by which the lava rose up from below. 



The argument is ingeniously put in these terms. " Had the fluid 

 matter been poured out on an inclined plane, the bed when consoli- 

 dated would have formed an elbow with the dike like the upper bar of 

 the letter F, instead of extending itself on both sides like that of a T 

 (Mem. pour servir, vol. iv. p. 149), and, moreover, the series of sheets 

 of lava would have been more numerous in parts of the mountain 

 farthest from the axis, for all the dikes which were feeders or sources 

 of lava would have poured their contents down the sloping cone and 

 never upwards." Although the rectangular junctions here alluded to 

 escaped my observation in 1828, and I have not revisited Etna since 

 M. de Beaumont wrote his account of them, I shall take the liberty 

 of offering a few comments on his statement of facts and method of 

 interpreting them, as they appear to me so extraordinary that I feel 

 at least entitled to demand, that the writer should acknowledge some 

 difliculties in which his theory would involve us. In the first place 

 I would ask, whence came the intercalated, incoherent and frag- 

 mentary beds ? M. de Beaumont can hardly escape the inference, 

 that they have been emitted from the same orifices as the dikes, if 

 these last were really the feeders of sheets of lava flowing out into 

 the atmosphere. But if the lapilii and scoriae were cast out at the 

 same points of eruption as the lavas, how could they possibly be of 

 the same thickness near the vents and at a distance from them ? If 

 an even plain had existed at the commencement of these fissure erup- 

 tions, it would soon have acquired an irregular surface, for larger 

 heaps of scorise would have been heaped up near the edges of the 

 supposed linear vents, than at greater distances from them. I may 

 also observe, that if vertical fissures gave vent originally at their upper 

 extremities to horizontal sheets of lava so as to form dikes, joining at a 

 right angle with the incumbent beds of lava, these dikes (fig. \0.a,b,c) 



