126 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juiie 23, 



and of Motta Sta. Anastatia, as well as those of Aci Reale and Trezza 

 near Castello di Aci, show the geological character of basaltic dykes 

 (accompanied near Patemo by a tufaceous conglomerate composed 

 of much altered fragments of secondary rocks). The direction of aU 

 those cliff-like outbreaks of basaltic rocks is perfectly the same as 

 that of the dykes of the Rocca, Giannicola, Musarra, &c. in the Yal 

 di Bove, and coincides, too, with the direction of that remarkable 

 fissure which runs from the crater of Etna to the Torre del FHosofo, 

 and further on to the Cistema. 



The fact of the frequent return of that remarkable direction, 

 S.S.W.-N.N.E., in so many dykes at the southern base of Etna, 

 and in the interior of the Yal di Bove, may be supported here by an 

 example which, moreover, presents a geological accident of high 

 theoretical interest. 



In order to bring what I am about to say on the fact represented 

 by fig. 11 (p. 127) in connexion vtdth that class of phaenomena of 

 which it forms an excellent example, we will start from the foot of 

 the lateral cone on the border of the Piano del Lago, the " Mont- 

 agnuola," iu a south-eastern direction, with the intention of examin- 

 ing the nature of the upper beds forming the sharp crest of the deeply 

 fractured southern wall of the Yal di Bove. The lava of the 

 Montagnuola is distinguished by the basalt-like black colour, as also 

 by the presence of olivine, labradorite, and augite in the interior of 

 the mass. Excepting this, it is very difficult to point out a good 

 character of difference between the rocks forming the beds of older 

 origin (of Etna) and those which broke out after the catastrophe of the 

 subsidence in which originated the Yal di Bove. All the lava-beds 

 of the Piano del Lago have an equally strong action upon the mag- 

 netic needle, when they are examined in the interior of the Cisterna 

 or Cisterella, as well as at the border of the Piano. The resemblance 

 to the modern lava is in general a striking quality of the uppermost 

 beds of the elevated walls surrounding the elliptical chasm of the 

 Yal di Bove. There seems to be an insensible passage from the 

 doleritic character of those upper beds into a more trachytic one on 

 the deeper-lying beds. Nevertheless the great confusion and inter- 

 sections which prevail among the projecting beds along the border 

 of the valley, in consequence of subsidences, reiterated eruptions, 

 and atmospheric agencies, make it a matter of great difficulty to study 

 such mineralogical passages step by step. The extraordinary variety 

 of felspathic lavas belonging to the trachy-dolerites, in which pyroxene 

 often makes its appearance, coincides with the same variety in the 

 numberless dykes by which the former are intersected. Amongst 

 those dykes I observed one in the usual form of an almost perpen- 

 dicular wall, trending exactly N.N.E. and S.S.W. It is of no con- 

 siderable height, but of great length. It was a very compact, fine- 

 grained, and pale-grey greystone of phonolitic appearance, sonorous, 

 and several feet in diameter. In the middle this dyke is split into 

 numberless plates, like slate of the finest quality, rising up per- 

 pendicularly. These central slates are very friable and sound like 

 glass. Towards the outer sides of the dyke the grey homogeneous slaty 



