Mr. PouLETT ScROPE OH the Geology of the Ponza Isles. 225 



In the island of Ponza, where alone these two classes are found together, 

 they are separated by horizontal strata of tufa, which probably belong to the 

 later rock, since they have been deposited after the subjacent trachytes had 

 suffered a considerable degree of denudation. The graystone is also in every 

 instance accompanied by its peculiar conglomerates. 



The first class of rocks bears a decided analogy to some of the trachytic 

 formations of Hungary, so minutely described by M. Beudant, and is perhaps 

 more nearly allied to them than to the more contiguous rocks of the same 

 family in Italy, France, and Germany. 



Thus the prismatic trachyte of Ponza corresponds very closely in mineral 

 composition to some varieties of the " trachyte" of the Hungarian groups. 

 It is however clearly distinguished from the predominant rock species of those 

 districts by the total absence of hornblende, which is so frequent and conspi- 

 cuous an ingredient in the trachytes of Hungary, as occasionally to bring them 

 very near to the characters of basalt. The Ponza trachyte differs also from 

 that of Hungary, as well as from most rocks of this family yet observed, in the 

 regularity and constancy of its prismatic configuration ; but this character is 

 of trifling moment in the comparison, since it mainly depends upon adventi- 

 tious circumstances ; the same mass of rock being in many instances found to 

 occur prismatic in one part and amorphous in another. 



Another peculiar feature of the Ponza trachyte is its ribboned structure, in 

 which as well as in its constant association with a semi-vitreous conglomerate, 

 it offers some analogy to the Perlite of Hungary. After an attentive perusal of 

 the great work of M. Beudant, and an inspection of the collection of specimens 

 brought by him to Paris, no doubt remained on my mind that the zoned 

 structure of the Hungarian perlite (semi-vitreous trachyte) and the lenticular 

 form of its lithoidal parts are owing to the substance of the rock having been 

 drawn out in the direction of the zones while possest of an imperfect fluidity, 

 and after the globular-concretionary separation characteristic of pearlstone, 

 had wholly or in part taken place. The motion was probably owing in this 

 case to the subsidence or flowing of the matter, in obedience to the impulse 

 of its own gravity, down a slightly inclined plane, (since the direction of the 

 zones is universally more or less horizontal,) as was the case with the vitreous 

 lava currents of Lipari, Volcano, Teneriffe, and Iceland *, which are ribboned 

 in the same manner, and pass repeatedly into pearlstone. In Ponza and Pal- 

 marola the zones are more frequently vertical than horizontal, and appear to 

 have invariably taken the direction in which the mass was impelled, from 



* Currents of obsidian of Von Buch, Sir G. Mackenzie, Dolomieu, &c. &c. 



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