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



it is not visible, on the eastern face of the island, beyond the Chiaja del 



Schiavone ; nor on the western, north of the Punta de' Faraglioni. 



The cliff's within this space are composed of the semi-vitreous conglomerate 



to a certain distance, beyond which the mass of the island consists solely of an 



anomalous rock of very variable and singular characters, and constant only in 



its extremely siliceous nature. From this circumstance I may distinguish it 



by the name of 



Siliceous Trachyte. 



In their external features, the escarpments formed of this rock, and exposed 

 to the erosion of the sea-spray and meteoric agents, strongly resemble the 

 siliceous burr-stone of the Paris Basin, having the same carious, hatched, and 

 angular surfaces. Their predominant colour is yellowish white or gray. 



Under its most general form this rock offers a white earthy and friable sub- 

 stance traversed by irregular concretionary parts of a compact texture, an ash- 

 gray colour, and often a flinty lustre. These usually constitute the greater 

 part of the rock. The earthy portions cut with the knife like fine chalk, which 

 they exactly resemble in outward appearance. Their whiteness is often of 

 the purest and most dazzling brilliancy ; and their detritus carried off by the 

 rain from the rocks in the interior of the island, is collected from the beds of 

 torrents, and made use of as whiting by the inhabitants. The compacter 

 parts seldom yield to the knife at all. Both appear to consist of nearly pure 

 silex, and are fused with the greatest difficulty by the blowpipe. They contain 

 a few disseminated crystals of mica, which are in general wholly or partially 

 decomposed into an ochry earth. The fissures of this rock are often lined 

 with ferruginous dendrites, or with a coating of delicate quartz crystals. 



By degrees the compact parts predominate still further, and inclose nume- 

 rous very perfect crystals of felspar, some of which are in the state of kaolin, 

 while others are glassy and brilliant, roundish grains of pellucid quartz, and 

 a few rare crystals of black mica. The basis of this porphyritic rock is close 

 grained, and exceedingly siliceous. It is heavy, solid, and tough ; of straight 

 and rather uneven fracture, a clear grayish white colour, and strikes fire 

 readily with steel. Through this rock the white earthy parts only appear 

 scattered in irregular patches, giving it a brecciated aspect ; and often dis- 

 appear altogether for a considerable space. At the northern extremity of 

 Ponza, west of the Punta dell' Incenso, this quartzose porphyritic trachyte 

 is traversed by an irregular vertical vein, from six inches to two feet in 

 diameter, which incloses where it is widest a number of fragments of the 

 same rock evidently broken off from the sides. The vein is filled by a black- 

 ish clay studded with cubical iron pyrites with a smaller proportion of copper. 



