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



Towards the upper parts of this extremity of the island, and particularly at 

 the summit of the cliifs which face the east between the Punta dell' Incenso 

 and the promontory marked S in the map, the siliceous trachyte passes into 

 another variety of rock, also of a fragmentary character, which wherever it 

 has been long exposed to meteoric action is rendered extremely carious, and 

 acquires a dead-leaf or iron-rust colour, with a dull earthy aspect. The frag- 

 ments generally appear fibrous like pumice, and are united together by a 

 siliceous cement. This cement resists decomposition, while the fragments it 

 envelops have decayed to a considerable depth into a sort of clay, which 

 mixes readily with water, and is therefore easily carried away by the rains ; 

 and hence arises the cavernous and corroded character of the exposed parts 

 of the rock*. The fragments themselves also decompose unequally, from 

 containing alternate hard and softer zones, and their fibrous aspect is in great 

 part derived from this circumstance. This rock in its unaltered state is 

 traversed by irregular fissures, which are lined with numerous groups of 

 quartz crystals, some measuring an inch in length, having frequently an 

 amethystine tinge, or coated with red oxide of iron. Some of the fragments 

 also appear to be agglutinated by limpid quartz and oxide of iron. 



Such are the different varieties of this singular rock ; all of which pass 

 insensibly into one another. It is not easy to determine the real relations of 

 the siliceous trachyte to the semi-vitreous conglomerate which succeeds it on 

 the south. At the western side of the island the former rock seems to overhe 

 the latter ; but in the clitfs on the opposite shore their position is reversed. 

 At each spot the limits of the two rocks are evanescent, and it is impossible to 

 say where one ends and the other begins. This uncertainty is perhaps occa- 

 sioned in part by a mutual penetration taking place along their common boun- 

 dary, but chiefly by the difficulty of distinguishing the white earthy base of 

 the conglomerate from the equally white and earthy parts of the siliceous 

 trachyte. 



The small cap-shaped islet called la Gahhia, which is divided from the 

 northern extremity of Ponza by a narrow strait, consists entirely of siliceous 

 trachyte, presenting the same varieties as in the main island. 



From the above details, which will I fear be found tediously minute, but 

 which were rendered necessary by the complex and singular nature of the 

 rocks to be described, it will appear that the trachytic rocks of the island of 



* The resemblance which it bears to the most cellular of the Paris burr-stoae arises from the 

 identity of the process which both rocks appear to have undergone. 



