PRODUCTION OP VOLCANIC DYKES. 485 



lent, though I am not aware that anyone dyke is to be found quite free 

 from leucite in large or small crystals. One dyke of those examined 

 by me (namely, 11 b) contains crystals of idocrase in notable pro- 

 portion. As respects the lithological character of the entire mass of 

 each dyke, the whole maybe divided into three classes : — 1. those of 

 dark-coloured compact and generally subcrystalline lava, which con- 

 stitute a large proportion of the whole ; 2. those of a much lighter- 

 coloured, softer, and very imperfectly subcrystalline lava of that 

 sort which Lyell has denominated " Graystone," of which material 

 immense dykes also occur in the escarpment of the Yal del Bove 

 (a few of the larger of these dykes in Somma show, though ob- 

 scurely, a tendency to a cross columnar structure, one which, in the 

 case of some of the very largest of these dykes of the Yal del 

 Bove, is magnificently developed, the hexagonal prisms being, in the 

 case of one very thick dyke examined by me upon the west side, 

 nearly horizontal and perfectly square to the sides of the dyke, 

 and in prisms perfectly detachable from each other, and from 

 ten to fifteen inches in diameter) ; 3. the third class on Somma 

 consists of thin dykes of highly compact material, dark-coloured, 

 and either gray or green, very hard, and with but little appearance 

 of even minute crystalline structure. 



The substance of these may be called phonolite in the grey, and 

 greenstone in the other dykes. In one of the triple dykes exa- 

 mined by me, viz. no. 11, a, b, and e, a and b are in absolute con- 

 tact with a mere joint of separation, and are nearly vertical, while 

 c appears to meet the others at a point a few feet below the Atrio, 

 diverging from them at an angle of about 45°, the intervening 

 wedge being filled in with volcanic conglomerate and pebbles. All 

 these three adjacent dykes are of different sorts of lava : a is largely 

 leucitic, and contains very small crystals of olivine, or perhaps of 

 sphene ; b is leucitic with crystals of pyroxene, and containing also 

 idocrase; while c is an extremely hard and absolutely compact 

 pyroxenic lava, like a greenstone, with a well-marked cross sub- 

 columnar structure. 



These three dykes would therefore appear to be either of different 

 ages, or derived from different sources of crater-contained lava, differ- 

 ing in composition and perhaps injected from very different depths. 

 While the jointing in all these dykes is generally more or less trans- 

 verse to their plane, evidences of a distinct laminated or slaty cleav- 

 age, sometimes with complete separation of the flakes, are perceptible 

 in some of the dykes which have been subjected to severe lateral 

 pressures, producing curvatures and twisting. The planes of lami- 

 nation are very generally inclined to the general plane of the dyke, 

 the strike of both more or less coinciding ; and this structure usu- 

 ally becomes evanescent at a few yards in length of the dyke, which 

 then returns to the normal transverse jointing. The dykes num- 

 bered 5, 6, 7, 10, 12, 13, 15 bis, 16, 22, and 25, all present more 

 or less distinct evidences of this structure, the crushing forces 

 producing which do not seem to have been always in a direction 

 normal to the lamination, but to have shattered the mass bv a 



