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



length, running- East and West, and the obtuser angle pointing to the North. 

 The surface has one general slope from the summit of the Capo del Arco, a 

 bluff promontory which forms the acute and western angle to the eastern 

 side, where a small port, almost the only landing place in the island, is exca- 

 vated from the indurated tufa rock. A cluster of habitations is grouped 

 around it. With the exception of this one point, the island is circumscribed 

 by perpendicular cliffs. Its physical structure is of the simplest character, 

 and corresponds exactly with that of the island of Procida, and of the neigh- 

 bouring hills which are so closely grouped together in the volcanic district 

 included between Naples, Pozzuoli, Cumae, and Aversa, and which are gene- 

 rally known to geologists under the name of the Phlegraean fields. 



The island consists of parallel and wavy strata of yellow and brownish tufa, 

 resting upon a massive bed of graystone, which rises to a height of more than 

 300 feet above the sea at the extremity of the Capo del Arco, and sinks gra- 

 dually towards the east below the water level. Though the remainder of the 

 island exhibits no rock but tufa, this slanting bed is without doubt prolonged 

 beneath it, and supplies the solid foundation of the entire mass. Indeed the 

 graystone which disappears on the south side at less than half its length from 

 the eastern promontory, is visible throughout the whole of the north-western 

 side. This rock is of a dark violet or purplish gray, fine-grained and compact, 

 strikes fire readily with steel, and contains small imbedded crystals of felspar, 

 augite, and mica. The upper part at the Capo del Arco is cellular, and even 

 scoriform at the surface of the bed. The lower is amorphous, or split into 

 massive blocks by irregular fissures, which are evidently but the result of the 

 retreat that accompanied its consolidation. The external surfaces of the rock 

 are exceedingly dark-coloured, almost indeed black; and this sombre colouring 

 together with its harsh forms give an unpleasing and savage effect to the cliff 

 that towers in gloomy majesty above a group of lesser rocks projecting from 

 its foot. The westerly swell, which at the time of my visit forced its way 

 with tremendous roarings into the caverns it has worn in these black, shape- 

 less, and scorified masses, added to the horror of the scene. The fable of 

 the dark dogs that barked round the waist of Scylla appeared to me more 

 appropriate to this promontory than to that less fearful one on the Calabrian 

 coast. 



The tufa of Ventotiene is interstratified with lapillo, or coarse fragmentary 

 pumice. It is incoherent in the higher parts of the island; but to the east, 

 and particularly wherever it approaches the sea-level, sufficiently indurated to 

 be quarried and shaped for building-stone. The loose strata contain many 

 fragments of granite and sienite, and of a graystone sometimes resembling a 



