228 Mr. PouLETT Scrope on the Geology of the Ponza Isles. 



that of Zannone. The pitchstone dykes of the former locality, and the highly 

 siliceous rocks which accompany the aluminite of the latter, are additional 

 features of resemblance to the Ponza formation. 



But independently of the analogies which may be traced in them to other 

 districts, the trachytes of the Ponza group present circumstances of great in- 

 terest in a geological view, from their novelty and anomalous nature. 



The principal of these it may be worth while to recapitulate. They are, 



1. The extremely regular and constant columnar division of an earthy tra- 

 chyte. 



2. Its ribboned structure, and the gradations which prove this character to 

 arise from the linear extension of the mass while in a state of imperfect liqui- 

 dity, coupled with a concretionary process. In fact, in this rock it is easy to 

 trace the operation of three distinct and consecutive processes, to which it was 

 subjected in the following order: viz. 1st, The concretionary arrangement of 

 its constituent minerals into spherolites of felspar imbedded in an impure and 

 partly siliceous base. 2ndly. The movement of the mass by protrusion from 

 below, accompanied by the tumefaction, elongation, and finally the confusion 

 and attenuation of the felspar globules, by the production of porous parts, 

 and the fracture and envelopment of some external portions which consoli- 

 dated more rapidly than the rest. Srdly. The separation into columns. Upon 

 reflection it will perhaps appear that these observations are of a nature to ex- 

 plain many of the most remarkable features of some trap-rocks, such as the 

 zoned and schistose structure of clinkstone ; and may even illustrate the simi- 

 lar appearances of elongation and contortion so frequent in gneiss and mica- 

 slate. 



3. The peculiar disposition of this rock ; from which it appears to have re- 

 peatedly broken through superincumbent beds both of semi-vitreous conglo- 

 merate, and of older trachyte ; with which it alternates in irregular and more 

 or less vertical beds, almost in the manner of the alternations of ditFerent va- 

 rieties of trap with conglomerate rocks in the transition series. 



4. The conversion of the rocks thus cut through, both conglomerates and 

 trachytes, to a greater or less depth from the planes of contact, into pitchstone, 

 generally porphyritic and passing both into pearlstone and obsidian ; — a fact 

 which may serve to throw some light on the hitherto obscure history of this 

 remarkable rock. In fact, the pitchstone of Ponza and Palmarola is thus 

 proved to be only a modification of trachyte, effected by a more or less incom- 

 plete fusion under considerable pressure and friction ; and appears to be pro- 



