Mr. PouLETT ScROPE OH the Geology of the Ponza Isles. 215 



On the south the graystone descends to a lower level*^ and sinks beneath 

 the sea in the Punta della Guardia ; on either side of this cape it may be seen 

 to he directly on the semi-vitreous trachytic conglomerate, which here exhibits 

 no appearance of chang-e at the plane of contact ; probably because the heated 

 mass having flowed from above down the slope of the hill, cooled superficially 

 as it advanced, and upon reaching this rock did not retain sufficient heat to 

 influence its texture. In fact, this is the habitual conduct of ordinary lava- 

 currents, which are seldom, — it may I believe be said never, — found to produce 

 any alteration of importance in the rocks which they meet with in their de- 

 scent. It is while the boiling matter is forcing its way from below through 

 the overlying strata which impede its expansion, (as was the case with all the 

 largest kind of dykes and protruded masses of the trap family,) and before it 

 has been enabled to part with the intense caloric that actuates its rise, upon 

 the easy terms afforded by the open atmosphere, that the rocks with which it 

 then comes in contact suff"er from its proximity ; and this, in a degree deter- 

 mined by their nature, the duration of the contact, the temperature of the 

 lava-mass, and other modifying circumstances. 



Westward of the Punta della Guardia, the graystone sends out into the 

 sea another low promontory called the Punta del Fielo, which forms the 

 southern limit of the Chiaja di Luna. Here some strata of loose tufa appear 

 broken through as well as covered by the contiguous rock. These are evi- 

 dently the prolongation of the strata seen beneath the cap of the hill from the 

 Chiaja di Luna. The whole mass of graystone is continuous, forming but 

 one entire bed, and apparently bearing the same relation to the somewhat 

 basin-shaped concavity occupied by the Harbour of Ponza, as the analogous 

 though smaller rock of the Monte Olibano, near Pozzuoli, to the crater of 

 the Solfatara. In fact, the disposition of the stratified tufa which clothes the 

 principal slopes of this basin, — a much safer criterion than mere form, — points 

 it out as the remnant of a large crater, as was judiciously observed by Dolo- 

 mieu. 



Graystone occurs in no other part of the island. 



Palmarola. 



The high and rocky islet of Palmarola lies about four miles and a half west 

 of Ponza and in a parallel direction to it. It is scarcely two miles in length, 

 and of a very unequal breadth. Like Ponza, this island seems to have lost 



* See Plate XXIV, fig. 3. 

 VOL. II. — SECOND SERIES. 2 F 



