Mr. PouLETT ScROPE ow the Geologj/ of the Ponza Isles. 233 



nature of graystone; while others, consisting almost solely of felspar, have 

 every right that can be derived from mineral composition to the denomination 

 of trachyte*. Both species are accompanied by tufas containing- much pu- 

 mice. Ischia is by means of Procida in almost immediate contact with the 

 Phlegraean fields of Pozzuoli and Naples, where upwards of thirty deformed 

 craters, thrown up by submarine eruptions upon a shallow coast, are still 

 distinguishable within a small space. The lavas of this district are chiefly 

 gray stone ; but some, as for example that of Astroni and the Monte Olibano, 

 are unquestionably trachytic. 



Further on rises the insulated cone of Somma and Vesuvius, which origi- 

 nally appears to have produced the same class of rocks, while its later lavas 

 are solely of a basalt, in which leucite entirely replaces the felspar, from the 

 elements of which it is probably in part composed. Some obscure deposits of 

 feldspathose rocks, with their associated tufas, appear at intervals within the 

 recesses of the eastern shore of the bay, as far as the Punta di Campanella. 

 Southward of this cape no traces of volcanic formations exist, I believe, nearer 

 than the Lipari Isles. 



Towards the North, the hilly group of La Rocca Monfina, rising above 

 Sessa and Teano, attests another powerful source of volcanic action. Like 



* The term trachyte was invented by Haiiy, and subsequently adopted by Daubuisson, Brongn. 

 iart, Brochant, &c., to designate a rock of peculiar mineralogical characters. An attempt 

 has since been made, probably through inadvertence, by M. de Beudant, and after him by 

 M. de Humboldt, to limit the name, thus appropriated, to rocks under particular circumstances 

 of geognostical position, these authors refusing the appellation to rocks identical in mineral com- 

 position with those to which they apply it, when they occur in the form of lava. currents, or cover 

 their conglomerates or tertiary formations. This restriction of a purely mineralogical term is not 

 only unwarrantable, according to all the legitimate principles of nomenclature ; but is certainly 

 founded on mistake, since a large proportion of the very rocks which are brought forward by 

 these writers as types of their trachytic formation, overlie their own conglomerates, as well as 

 basalt, and freshwater and tertiary limestones, and frequently occur in the form of lava-currents ; 

 — ex. gr. in the Mont Dor, Cantal, Euganean Hills, Mont Amiata, Monti Cimini, Teneriffe, 

 Leeward Isles, &c. &c. 



In fact, just as granite, gneiss, porphyry, and serpentine, are denominations purely relating to 

 mineralogical character, and as such equally applicable to the oldest as to the most recent of these 

 rocks, so must the terms Trachyte and Basalt be allowed to remain independent of all question as 

 to age or geognostical relations. It is obvious that the determination of the mineral nature of a 

 rock must precede any attempt to find its place in the geological series (since it is only by these 

 characters that it can be distinguished from or compared with the other rocks with which it is 

 associated). The primary name must therefore be purely mineralogical ; and if the appellations 

 of trachyte and basalt are absolutely to be confined to a geological meaning, it behoves the authors 

 who make this change to substitute in their room some other primary names, which may serve to 

 designate rocks of their peculiar character in whatever situation they may be found. 



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