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



teristic of the other rock. A similar conglomerate occurs on other neighbour- 

 ing parts of the island^ but never shows any signs of disturbance, always 

 overlying the rocks already described, and adapting its strata to the irregula- 

 rities of their surfaces. 



In the instance under consideration, the disposition of these fragmentary 

 strata proves them to have been deposited subsequently to the formation of the 

 prismatic trachyte against which they lean on either side ; apparently filling 

 a deep chasm between these two masses, which in all probability was the re- 

 sult of some violent convulsion. This observation may appear superfluous, 

 but is called for by the resemblance of the nearly vertical bed of trachyte on 

 the right side to a dyke or intruded mass. Its evidently posterior origin to the 

 tufa which bounds it on one side, proves that it cannot have been really pro- 

 duced in that manner. 



The natural section of this bed of tufa presents an absolutely vertical wall 

 about 300 feet high ; which being sapped by the waves whenever the westerly 

 winds blow directly into the cove, peels off at intervals in massive laminae, and 

 thus continually offers a fresh surface. 



Beds of a similar conglomerate, probably once a continuation of the same 

 mass, are observable on the south side of the cove. One portion of them has 

 been deposited on the uneven and concave surface of a mass of prismatic tra- 

 chyte, all the strata following the sinuous outline of the latter rock. Another 

 set of strata lie horizontally across the edges of two neighbouring masses of 

 the prismatic trachyte and semi-vitreous conglomerate, as well as of the former 

 waved tufa, all of which must have simultaneously suffered a certain degree of 

 denudation before its deposition. These beds of tufaceous conglomerate 

 contain large fragments of all the varieties of trachyte mentioned above, which 

 have the aspect and forms commonly met with in the ejected blocks of volca- 

 nic conglomerates. 



Another series of similar strata, and equally to all appearance a continuous 

 part of the same deposit, shows itself on the opposite side of the island, forming 

 a line of cliffs which range southward from the Punta della Madonna, at the 

 entrance of the harbour of Ponza*. These also rest indifferently upon the 

 prismatic trachyte and semi-vitreous conglomerate. No alteration is ever ob- 

 servable in them at their boundary with the former rock ; their separation 

 from the latter is equally decided and complete. Both had evidently been 

 consoUdated before their deposition, and have remained undisturbed ever 

 since. The same tufa crops out on two or three other points towards the base 



* See Plate XXIV. fig. 1. 



