THE CEUST OF THE EARTH. 



feet in the height of its bounding precipices, which in most places- 

 are nearly perpendicular. This remarkable area appears to have 

 resulted from the giving way and subsidence of part of the crust 

 of the volcano, from some violent action in the interior, which 

 occasioned the sudden removal of an enormous mass of mineral 

 matter. This plain is ^ncircled by subordinate volcanic moun- 

 tains, some of which are covered by forests, while others are bare 

 and arid like many of the cones of Auvergne. The walls or cliffs 

 surrounding this depression are formed of successive layers of lava 

 of variable thickness, with interposed beds of tufa, ashes, and 

 igneous conglomerates of different colours and degrees of fineness. 

 They slope downwards towards the sea at an angle of from twenty 

 to thirty degrees, and have evidently been formed at various 

 intervals by successive eruptions from the top of the mountain, 

 and were continuous before the subsidence took place which gives 

 this region its present character. 



The perpendicular sides of this natural amphitheatre are every- 

 where marked by vertical walls or dykes, which not only intersect 

 the concentric sheets of lava and tufa, but, standing out in bold 

 relief, like prodigious buttresses, impart a most extraordinary 

 character to the scene ; the greater induration of these intruded 

 dykes having enabled them to resist the denuding action which 

 has removed the less coherent pre-existing erupted materials. 

 These buttresses are from two to twenty feet in thickness, and 

 being of immense height, are extremely picturesque ; some of them 

 are composed of trachyte, and others of blue compact basalt with 

 olivine. The surface of the plain is wild and desolate in the 

 extreme, presenting the appearance of a tempestuous sea of liquid 

 lava, suddenly congealed. Innumerable currents of lava are seen 

 piled one upon the other, some of which terminate abruptly, while 

 others have extended across the Val, and descended in cascades 

 into the lower fertile regions, where they are spread out in sterile 

 tracts amid the vineyards and orange groves.* 



147. A section of Mount Etna, extending north and south 

 between Catania on the south, and Taorminia on the north, is 



shown in fig. 62 ; the east, upon the slope of which the cavity in 

 question is observable, being presented to the observer. 



* See Captain Basil Hall's graphic description of a visit to the Val del 

 Bove, "Patchwork," vol. iii. p. 31. 

 110 



