Mr. PouLETT ScROPE on the Geology of the Ponza Isles. 231 



in producin<^ this pseudo-conglomerate structure ; and indeed it may be con- 

 ceived without difficulty^ that should such a process take place before the mo- 

 tion of the mass had entirely subsided^ or a renewal of motion occur during 

 the action of the elective process, the concreted parts, offering an impediment 

 to the motion of the surrounding liquid, would be broken up and enveloped 

 in such a manner as to participate more or less in the double character of no- 

 dular concretions and imbedded fragments. 



7. The occurrence of a vertical vein of cupriferous iron pyrites in the sili- 

 ceous trachyte, having many characters in common with ordinary metalli- 

 ferous veins, such as the association of the ore with black clay and quartz 

 crystals, is a fact which would have been more striking a few years back, be- 

 fore it had been recognized that trachyte is frequently the matrix of the rich- 

 est metals ; as for example, of the gold of Villalpando in Mexico, of Konigs- 

 berg and Telkebanya in Hungary, &c. In the present case the ore appears 

 to have been produced by sublimation, since it penetrates the sides of the 

 fissure and the included fragments, exactly in the manner of the specular iron 

 so common in the clefts of volcanic rocks. The quartz crystals are of contem- 

 poraneous formation. 



In Ponza the trachytic formation is separated from the incumbent rock 

 which has been designated as graystone, by strata of loose conglomerate, 

 chiefly composed of pumice, and which may belong in part to both the under 

 and overlying formations, since their disposition in the Chiaja di Luna proves 

 them to have been deposited at two different periods. In Palmarola the stra- 

 tified tufa must be supposed to belong to the trachyte, since in this island 

 graystone is wanting. 



The mineralogical characters by which this last rock (graystone) distin- 

 guishes itself from the subjacent trachyte have been already sufficiently enu- 

 merated. Its frequent occurrence in Ischia, Procida, and the volcanic district 

 round Naples, has been also mentioned ; and in the Scoglio della Botte and 

 the islands Ventotiene and San Stefano, we see a series of similar rocks geo- 

 graphically connecting these localities with that of the Montagna della 

 Guardia in Ponza. The huge mass of graystone of this mountain, resting 

 at a great height upon stratified tufa on that face of the hill which overlooks 

 the basin of the harbour, and on the other side following the inclination of 

 these strata in a rapid descent below the level of the sea, has precisely the 

 figure and disposition of a lava-current. 



The bed of graystone which forms the substratum of Ventotiene has even 

 more decidedly the form and aspect of a mass which has flowed down an in- 



VOL. II. — SECOND SERIES. 2 H 



