Mr. PouLETT ScROPE ofi the Geology of the Ponza Isles. 211 



Ponza are disting^uishable into two masses; the one an earthy trachyte, pretty 

 uniform hi structure and composition, associated with a conglomerate of pu- 

 mice and semi-vitreous trachyte, which is almost universally converted into 

 pitchstone near the former rock : the other a very quartzose porphyritic tra- 

 chyte of a novel and anomalous character, which is also accompanied by 

 breccias and conglomerates equally siliceous. 



The relative positions and mutual interference of these varieties will be best 

 understood from the accompanying outline sketches of the principal cliff ranges 

 of the island*. 



The most interesting of these natural sections is that which circumscribes 

 the Chiaja di Luna, a deep semi-circular cove opening to the west, and cor- 

 responding to the harbour of Ponza, a similar indentation of the eastern 

 shore, from which it is only separated by a narrow and rocky isthmus. Across 

 this the Romans, when masters of Ponza, effected a communication for land 

 carriage between the opposite bays by means of an arched gallery cut through 

 the rock of prismatic trachyte and lined with reticulated brick-work. This 

 passage still remains entire, and carries off the waters that drain down the 

 concave slope of the Montagna della Guardia towards the port, which would 

 otherwise do considerable injury to the town and quays, by wearing away the 

 friable conglomerate on which they are chiefly built. 



The cliffs forming either horn of this crescent (the Chiaja di Lunaf ) pre- 

 sent the usual irregular intermixture of the prismatic trachyte and semi- 

 vitreous conglomerate ; the latter invariably passing into pitchstone towards 

 its line of contact with the former. The central part of the curve is occupied 

 by a deep bed of loose earthy tufa (trachytic conglomerate););. Both the 

 upper and lower parts of this mass contain coarse fragments of pumice and 

 compact trachyte ; disclosing its stratification, which in the intermediate bed 

 is not perceivable from the uniform fineness of the fragmentary matter. This 

 rock contains a few broken crystals of felspar and mica, and a large propor- 

 tion of magnetic iron-sand. Its characters are very distinct from those of the 

 semi-vitreous conglomerate already described ; it possesses little or no cohe- 

 rence, and incloses none of those blocks of glassy trachyte which are charac- 



* See Plates XXIV and XXV. 



+ See Plate XXIV. fig. 2. 



X Tufa is the trivial name adopted by the Italian geologists to designate all volcanic conglo- 

 merates of a feldspathose nature, which are composed principally of pumice. The conglomerates 

 of basalt or leucitic rocks they term peperiuo. The first term is totally distinct in meaning from 

 tufo, which is never applied to any other rock than calcareous tuff, and is rarely used at all, its 

 synonyme travertino being generally preferred. 



