Mr. PouLETT ScROPE on the Geology of the Ponza Isles. 209 



accompanied by crystals of quartz. This ore, as well as the crystals, is 

 attached in greatest quantity to the sides of the vein and the surfaces of the 

 fragments it incloses, coating them over with a metallic crust, and impregna- 

 ting the substance of the rock to a certain distance with disseminated pyrites. 

 A brown and yellow clay iron ore also encrusts the sides of the vein in some 

 spots, and stains the rock to which it is attached, to the depth of a few inches. 

 The washing of this vein by the sea-spray and rain clothes its exposed surfaces 

 with a coating of green carbonate of copper. 



On the west side of Ponza, and particularly within the Chiaja delle Forne, 

 this siliceous trachyte presents the appearance of a coarse breccia. Irregular 

 masses of a fragmentary aspect seem to be cemented by a less compact, and 

 often very earthy basis, into which however they may be often observed to 

 throw out venous ramifications, or pass by insensible gradations. If, there- 

 fore, they were originally fragments, most of them must have been enveloped 

 before they were entirely hardened, as was conjectured above in the case 

 of the siliceous and brecci-form parts of the prismatic trachyte. 



These blocks are generally ribboned or mottled with blue, brown, red, 

 yellow, light-green, white, and an infinity of intermediate tints. Their texture 

 is usually close, fine grained, and flinty. They also possess the lustre and 

 general aspect of flint ; sometimes the grain approaches to that of chalcedony, 

 and the cavities are lined with a mammillated coating of this substance which 

 graduates into the surrounding rock. Imbedded plates of mica and imperfect 

 felspar crystals occur in them. In the earthy parts the mica is usually decom- 

 posed into rusty specks. This rock rings like glass under the hammer, and 

 flies into exceedingly sharp-edged fragments, translucent at the edges. 



It is in general heavy and dense ; but in some parts becomes light and 

 porous ; the cells are then elongated in parallel lines. The crystals of felspar 

 and mica appear drawn out in the same direction ; the former broken up and 

 half fused. In this state it appears passing into pumice, but is equally silice- 

 ous with the compacter parts. It is often intersected by veins of almost pure 

 quartz of a milk-white colour ; some of these are 3 or 4 inches in width, and 

 are cancellated, or formed of plates crossing one another, so as to leave angular 

 spaces between them. In disposition and aspect they are very similar to the 

 quartz veins of granite and gneiss. Where any fissures occur, they are lined 

 with quartz crystals. Amongst the massive blocks inclosed in this brecciated 

 rock, I observed one of extraneous origin. It is granitoidal, composed of 

 glassy felspar, quartz, and a very little mica. This fragment left its impres- 

 sion in the siliceous rock, on being detached, while the other masses of frag- 

 mentary appearance wifl not separate along their seeming boundary lines. 



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