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



\ng action of t'le waves wherever the component rock was more friable than 

 that of the intervening promontories. When these points of extreme erosion 

 accidentally correspond on either side, the island is very nearly divided by the 

 junction of the two opposite coves. This occurs to a remarkable degree on 

 two different points of the island's length, in which its transverse diameter does 

 not exceed a few hundred feet. It is probable that in no great length of time 

 a complete separation will be effected on both these spots ; one of which is the 

 narrow strip of land that divides the cove called Chiaja di Luna from the har- 

 bour of Ponza ; the other, a still narrower ridge which backs the Cala d'lnfer- 

 no, and is now continually sapped by the breach of the sea on either side. 



Such a disjunction appears already to have taken place in more than one 

 instance at the northern extremity of the island, where a narrow channel cuts 

 off from it the littla circular islet called La Gabbia ; and further on in the 

 prolongation of the same direction, (which is that of the longitudinal extent 

 of the main island,) three or four insulated rocks rise above the water-level, 

 identical in constitution with the nearest extremity of Ponza, and connected 

 with it by a reef which may be traced by soundings, and seen on many points 

 when the sea is calm. The island of Zannone stands at no great distance 

 further on in the same line, and forms the extremity of this range of subma- 

 rine heights. The proofs of rapid and continual destruction visible in the 

 main island of Ponza, render it probable that the remaining insular eminences 

 of this chain were once united even above the sea level. 



The island does not offer a single uninterrupted and central ridge or axis 

 with a gradual slope on either side, as is often the case with such strips of ele- 

 vated land. Its present form rather leads to the supposition that it once consisted 

 of two parallel but irregular ridges, branching off from the massive eminence 

 which terminates the island to the south. Of these ridges the westerly one is 

 almost wholly worn away by the violence of the swell to which this quarter is 

 almost incessantly exposed. Tlie few more durable fragments that remain of 

 it, project as capes into the sea, rising in height from the interior of the island 

 to their extremity, where they terminate in an abrupt cliff, thus reversing the 

 general figure of ordinary promontories. The eastern range of hill not 

 being subjected to so constant an erosive force, is less degraded. There are, 

 however, few points on which any portion of its seaward slope remains, the 

 perpendicular escarpment of its water-worn face dropping in general directly 

 from the acute edge of the opposite or landward slope. 



A number of pointed and picturesque rocks of columnar trachyte rising 

 from the sea on either side of the island, at short distances from its steep cliffs, 

 add to the proofs of its greater original width, in the same manner as those 



