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



detached blocks, which prevent the approach of a boat with any swell except 

 on a few points, it is not easy to put every rock of the island to the test of 

 the hammer. My examination was also limited in>time by the rapid approach 

 of a hurrasca, which drove me hastily back to the port of Ponza for shelter. 



The boundary-line of the limestone and superposed trachyte is effectually 

 concealed on either side by a large talus of debris. The limestone is of a 

 bluish gray colour, hard, of semi-crystalline grain, and intersected by nume- 

 rous veins of calcareous spar, which constitute a large proportion of the mass. 

 I could find no organic remains. It has all the characters of transition lime- 

 stone, and appears to correspond with that of the Monte Circello, a vast 

 mountainous rock, (once the island of Circe, but now united to the continent 

 by the flat levels of the Pomptine marshes,) which rises exactly opposite 

 Zannone at a distance of 10 miles, and has been described by Brocchi, as 

 differing from the ordinary Apennine limestone, and decidedly transition. 

 The fragment of this rock which forms the Capo Negro in Zannone, exhibits 

 highly inclined strata, directed W. N. W. by E. S. E., and dipping towards 

 the Italian coast *. 



The loose blocks which have fallen from this limestone rock towards its 

 junction with the trachyte, and make up the talus by which the line of contact 

 is hidden, differ in character from the rock which forms the extreme point of 

 Capo Negro. Their colour is a dark reddish-gray approaching to black ; they 

 are no longer traversed by calcareous spar veins, effervesce with the greatest 

 difficulty, and appear to the lens to consist entirely of a close aggregation of 

 cubical crystals, which are most perfect within the fissures and small cellular 

 cavities that penetrate the rock. There can therefore be little doubt that the 

 limestone has been converted into dolomite to a certain distance from its 

 contact with the overlying trachyte, to the protrusion of which it is difficult 

 not to attribute the elevation of this insulated fragment of a transition forma- 

 tion. The frequent repetition of an analogous circumstance in the Alps, where 

 enormous masses of Alpine limestone are rendered crystalline and penetrated 

 with magnesia by the proximity of augitic basalt, and a secondary granite 

 with which it is associated, may be urged in support of this hypothesis f . 



* See Plate XXV. fig. 6. 



t In the Tyrol. See " Lettre de M. Leopold de Buch a M. A. de Humboldt, renfermant le 

 Tableau Geologique du Tyrol meridional." I take this opportunity of mentioning that I have 

 been enabled to verify the observations made in this locality by Count Marzari, as to the positive 

 superposition of a true granite to secondary conchyliferous limestone, and a sandstone with gypsum. 



The circumstance is most distinctly visible near Predazzo in the Val di Lavls. The upper 

 formation of limestone (which is probably Jura) is converted into coarse granular dolomite re- 



