1858.] ABICH ^ETNA. 127 



mass becomes more coherent, and passes into a black pitchstone- 

 like crust, which covers the sides of the dyke with a regular layer of 

 a more or less glassy substance, some inches thick, and as black as 

 coal. On the outside of the dyke the black glassy crust showed on 

 its uneven surface, and imbedded in it as in a paste, a great quantity 

 of small and great fragments of the rocks composing the horizontal 

 beds through which the dyke had passed. The magnetic power 

 of this dyke was considerable. Advancing still more along the crest 

 of Monte Zoccolaro, and bending a little on a more gentle slope of a 

 detached part of that enormous ridge towards the valley, I was 

 struck by the curious phaenomenon of which fig. 11 gives a very 



Fig. 11. — Grotta del Subi Legno, on the side of Monte Zoccolaro. 



true representation. A dyke of a mean diameter of 6 feet, consisting 

 of a compact felspathic lava, and trending N. 35° W., suddenly stops 

 the path. The surface of the outside is scorified, exhibiting a 

 smoothly rounded kidney-shaped crust. When turning round that 

 curious massive wall, which terminates at the crest in a kind of 

 pad, overhanging a little towards the south-eastern side, I perceived 

 that there was an immediate passage from the mass of the perpen- 

 dicular dyke into a regular layer or bed of the same stone towards 

 the eastern side. This layer, from its beginning at the top of the 

 dyke, went down with a mean inclination of 25°, and regained an 

 almost horizontal position from 6° to 0. The masses which once 

 fitted the corner between the dyke and its detached layer are partly 

 removed, having given way to a great extent by lateral subsidence. 

 The cavern thus formed is somewhat spacious and has the name of 

 *' Grotta del Subi Legno;" the physical nature of the roof of this 

 cavern is that of a half-smolten scorified lava ; great stalactites in 

 huge masses, hanging down from the roof, correspond to former 

 depressions in those fragmentary layers over which the lava, pouring 

 out of its perpendicular channel, had first to flow. That lava- 

 stream shows how lava- currents — suppose they are given out by 

 rents of considerable length — are capable of forming regular layers, of 

 even 25° inclination, without the slightest difference in the internal 

 structure of the rock both of the dyke and of the layer. On its 

 outer surface this lava-bed shows the same scorified appearance as 

 on the surface visible in the interior of the cavern. 



The nature of the mass which forms the dyke shows some in- 

 teresting peculiarities. The rock is compact and highly crystalline. 

 In the axial region a spongy zone of pores of considerable size runs 

 upwards; these pores are hollow, and of the same appearance as 



