454 Geological Society. 



Capt. Parry devoted much of their time to the collection of 

 the natural productions of the regions which they visited. 



A curious Papilio was exhibited by A. MacLeay, Esq. 

 Secretary, presenting the forms and colours, we believe, of 

 the Laodocus and Polycaon of Fabricius, which therefore 

 are doubtless the male and female of a single species. The 

 specimen has the wings of the former on one side, and those 

 of the latter on the other. 



GEOLOGICAL SOCIETY. 



May 21. — The reading of the paper " On the Geology of 

 the Ponza Islands in the Mediterranean ;" by George Pou- 

 lett Scrope, Esq. M.G.S., was concluded. 



The Ponza Islands lie off the coast of Italy opposite Ter- 

 racina and Gaieta. They consist of Ponza (anciently Pan- 

 dataria), Palmarola, and some islets; Ventotiene and San Ste- 

 fano connect them with Ischia. The harbour of Ponza is 

 excellent. Dolomieu's Memoire sur les Isles Ponces excited 

 curiosity, but is too general to satisfy it. These islands are 

 composed of rocks of the trachytic series; and presenting 

 fine sections along their coasts, enabled the author to clear 

 up many doubts and errors which the mere investigations of 

 inland localities have caused to be affixed to this formation. 



The Isle of Ponza is long and very narrow, and is eroded 

 by the sea into deep concavities. Harder masses left along its 

 shores show that it once was broader, and protruding ledges 

 mark its former connexion with Quannone and La Gabbia. 

 Prismatic trachyte, variously coloured and disposed, forms 

 the ossature of the island. It is constantly accompanied by, 

 and alternates with, a semi-vitreous trachytic conglomerate 

 formed of minute pulverulent matter inclosing fragments of 

 trachyte. The prismatic trachyte seems to have been forcibly 

 injected through the conglomerate ; and wherever it touches 

 the latter, its earthy base is converted from two to thirty feet 

 deep into a pitchstone porphyry ; sometimes it becomes a 

 pearlstone, at others incloses a true obsidian. These rocks 

 are connected with a siliceous trachyte resembling in ap- 

 pearance the siliceous buhrstone of Paris. Resting on the 

 semi-vitreous trachyte and forming the base of the Mon* 

 tagna della Guardia is a rock 300 feet thick, which the author 

 distinguishes mineralogically from common trachyte, and 

 proposes to call Greystone. 



In Jannone the trachyte overlies a limestone which Broc- 

 chi describes as transition limestone: at the point of contact 

 this latter becomes dolomite. Having described the whole 



of 



