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



saving annually a very considerable number of vessels and lives. During one 

 day that I was myself confined to the island by hard blowing weather, no less 

 than fifteen vessels, occupied in different branches of the coasting trade, put 

 in for shelter*. 



These islands were imperfectly described by Sir W. Hamilton in 1785, in 

 the Philosophical Transactions. Dolomieu examined them in the subsequent 

 year, and published his observations in considerable detail in his " Memoire 

 sur les Isles Ponces." But, at that period, geological investigations were 

 almost solely confined to the mineral characters of rocks, or to crude spe- 

 culations upon their origin ; very little attention being paid to their geogno- 

 stical relations. Owing partly to this cause, and in a great measure to the 

 changes that have since taken place in the nomenclature of the science, few 

 facts of any importance are to be gleaned from the accounts of either writer; 

 and consequently we find these islands unnoticed in most general works on 

 geology : the vague descriptions of the French travellers being perhaps mis- 

 trusted, while no more definite or recent observations have been made public 

 by any other geologist. 



Still it was known from the account of Dolomieu and from the specimens 

 brought by him to Paris, that these islands consist principally of trachyte, and 

 contain some remarkable resinous and quartzose varieties of this rock, the 

 relations of which became an object of geological interest. It was also known 



* A depot of convicts is established at Ponza, and another on the little rocky isle of San Ste- 

 fano, an appendage to Ventotiene. In the latter establishment the prisoners occupy a large circular 

 building raised for the purpose ; but in the former they are confined to the number of between 

 500 and 600 in a cavern excavated by the Romans, who probably made use of it for the same 

 purpose, since these islands are known to have been occasionally made the place of exile for state 

 prisoners". Stone-quarries appear to have been in frequent use among the ancients as prisons'*. 

 When the health of prisoners was entirely overlooked, and security and ceconomy the only 

 objects considered, such crypts or grottos offered considerable advantages, and many appear to 

 have been excavated with a view to such an ultimate adaptation. This den, for it deserves no 

 other name, has but two narrow spiracles cut through the rock to the surface of the hill above, by 

 which either light or air can be admitted, and these are closed at night. Indeed from the nar- 

 rowness and vertical depth of these openings, little exchange can be supposed to take place be- 

 tween the foul air below, and the purer, drier, and therefore lighter atmosphere without. Unless 

 we suppose the porous trachyte and tufa forming the walls of the cave to be permeable to atmo- 

 spheric air, it is difficult to account for the fact, that, whatever disease is engendered in so foul 

 a dungeon, still the air remains respirable to so many human lungs, notwithstanding the impedi- 

 ments by which its renovation appears to be opposed during the day, and the apparently total 

 obstruction to it at night. 



» Vide Tacit. Ann. lib. i. cap. 53. et lib. xiv. cap. 63. Sueton. in Tib. cap, liv. et in 

 Caligul. cap. xr. '' Fide Thucydid. lib. vii. cap. 86 et 87. 



