Mr. PouLETT ScROPE 071 the Geology of the Ponza Isles. 227 



to the " porphyre trachytique avec quartz " of Hungry, in its porphyritic 

 character, compactness, quartzose nature, and the chalcedony that hnes its 

 cavities. Its veins of nearly pure quartz, groups of amethystine crystals, and 

 the brecciated aspect of its flinty parts, bring it nearer to the porphyre mo- 

 laire of that country. M. Beudant, indeed, mentions these tvv^o species of 

 trachyte as passing mutually into one another, and therefore, as in Ponza, 

 mere varieties of the same rock. Lastly, the overlying conglomerate strata of 

 Ponza and Palmarola, almost entirely composed of triturated pumice, corre- 

 spond both in situation and nature to the " conglomerats ponceux " of the 

 Hungarian groups. 



Little analogy exists between the rocks of the Ponza isles and the trachytic 

 mountains of central France, beyond an occasional rather close similitude of 

 mineral composition, and those other points of resemblance that necessarily 

 arise from their common volcanic character. 



The trachyte of the latter district is disposed for the most part in vast beds 

 dipping gradually away on all sides from a common centre of eruption ; and 

 alternating horizontally with one another, with varieties of basalt and gray- 

 stone, and with their conglomerates, which latter are in no way altered at the 

 surfaces of contact ; while the analogous rocks of the Ponza group occur in 

 irregular dyke-shaped masses, which interfere with and interrupt their con- 

 glomerates in the most arbitrary and irregular manner, at the same time con- 

 stantly effecting a change in their texture to a certain depth from the plane of 

 contact. These differences will be satisfactorily accounted for by the re- 

 flection that the latter formation is evidently of submarine origin, while the 

 trachytes of Prance, which are superposed to a primitive platform averaging 

 2,800 feet above the sea, and rise themselves to more than double that eleva- 

 tion, must, in all probability, have proceeded from volcanic vents acting under 

 the pressure of the atmosphere alone. 



The other trachytic formations of Italy are so little known, that it would be 

 almost useless to bring them forward as objects of comparison ; and indeed 

 from their inland situation the examination of their structure is attended with 

 all the difficulties noticed in the commencement of this paper. It may be 

 mentioned, however, that the two groups of the Monti Cimini and Monte Ami- 

 ata appear to agree with those of France in the characters of disposition, and 

 therefore differ in this respect from the Ponza rocks, which they resemble 

 only imperfectly in mineral constitution. The trachytes of the Euganean 

 . Hills and of La Tolfa, on the contrary, show themselves, like those of the 

 islands in question, in enormous vertical masses breaking through older for- 

 mations, which in the last-quoted instance is a transition limestone similar to 



