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



San Stefano. 



East of Ventotiene, and at the distance of rather more than a mile, lies an 

 islet of this name, which is made use of as a depot for convicts by the Neapo- 

 litan government. It is nearly circular, less than half a mile in diameter, and 

 surrounded by steep and craggy cliffs. In structure it resembles Ventotiene, 

 consisting of a single mass of graystone, rising from 150 to 200 feet above 

 the sea, and surmounted by a few strata of loose tufa. The upper part of the 

 rock is exceedingly cellular; the lower has a rude tendency to the columnar 

 configuration. The composition of the rock is similar to the graystone of the 

 larger island. 



The surface of San Stefano is deeply indented by two shelving ravines, 

 opening on opposite sides : these I conclude to be the two craters spoken of 

 by Dolomieu- It is, however, probable, that if any crater was originally 

 formed during the protrusion of this mass, the loose and fragmentary parts of 

 its circumference must have been destroyed by the degrading action of the 

 sea. The few conglomerate strata that remain, appear to have moulded them- 

 selves on the irregular surface of the bed they rest upon, and are not suffi- 

 ciently abundant to point out the position of the orifice of production. The 

 proximity of this is, nevertheless, attested by the scoriform character of the 

 graystone rock. A calcareous sandstone similar to that described above in 

 Ventotiene, is found on the surface of this smaller island, and suggests the 

 same observations. 



From the above details on the constitution of this group of islands, it will 

 perhaps appear that they afford scope, as was anticipated, for observations of 

 considerable geological interest. It remains therefore to point out the analo- 

 gies they present to the volcanic formations of other neighbouring countries, 

 and recall the particular facts which tend to elucidate the yet obscure history 

 of these rocks. 



In a general view it is impossible to overlook the decided separation of the 

 rocks which compose these islands into two classes, apparently belonging to 

 two different ages (in this locality at least), viz. 



1. The trachytes with their accompanying conglomerates ; and 2ndly, those 

 rocks, which, without stopping to discuss the question as to their also belong- 

 ing or not to the family of trachytes, have been mentioned, to avoid present 

 discussion, by the name of graystone. 



