Mr. PouLETT ScROPE Oil the Geology of the Ponza Isles. 201 



brown, ochry brown, or blackish blue tint. The surface of the columns, and 

 consequently the natural face of the rock, is of an ochry brown, apparently 

 produced by the infiltration of an oxide of iron. The ferruginous matter has 

 frequently penetrated to the interior of the prisms, producing rovvs of concen- 

 tric stripes, usually parallel to the outer surface, and wholly independent of 

 the direction of the other streaks or zones, mentioned above. The latter di- 

 stinguish themselves by alternations of different texture as well as colour, the 

 darker layers being universally more compact and fine-grained, as well as 

 harder than the intervening parts of a lighter tint. 



These zones traverse uninterruptedly a great extent of rock, without any 

 regard to the position of the columns, and generally cutting them at some angle 

 to their axis ; thus indicating, that a partial modification of internal structure 

 in the substance of the rock, preceded its separation into prismatic concretions. 

 They are not necessarily straight, but frequently waved or contorted in con- 

 centric curvatures, like the stripes of certain varieties of marbled paper ; and 

 this both on the large and small scale. The thickness of the streaks varies 

 from some inches to that of the finest thread. This variegated appearance is at 

 times exchanged for one in which the rock assumes the character of a breccia 

 of fragments of different colours and grain embedded in a zoned basis. This 

 latter variety generally occurs towards the extremity of the mass, and where 

 it loses its prismatic division. 



This species of trachyte usually exhibits an earthy-granular base, consist- 

 ing of felspar, with numerous imbedded plates of mica, hexagonal or rhom- 

 boidal, but seldom entire, and imperfect half-fused crystals of glassy felspar. 

 It is porous, particularly the more earthy and lighter-coloured laminae, and the 

 direction of the porous parts as well as of the vacuities themselves, coincides 

 with that of the stripes. The dark-coloured zones are not only of a finer and 

 closer grain than the intermediate layers, but harder and apparently more sili- 

 ceous ; in some places they become so compact, as to communicate to the 

 rock a sharp and splintery fracture approaching to conchoidal, and a flinty 

 aspect. This is particularly the case, where it resembles a breccia. On these 

 points the rock flies, when touched by the hammer, in the manner of flint, and 

 rings like glass. The imbedded fragments are often, not always, equally 

 siliceous with the base, and so firmly cemented to it as never to separate at 

 the line of contact. They seem to be portions of the zoned or variegated 

 trachyte, broken off* by friction before the whole was consolidated, and enve- 

 loped in the parts which still remained liquid*. Where the alternate zones 

 are most contrasted in colour and texture, the whiter and more friable ones 



* In the manner of the brecciated marbles, jaspers, &c. 



2d2 



