298 J. W. Judd— On Volcanos. 



Fig. 8. — A. pachysoma, Gardner. From a specimen in author's cabinet. Grey Chalk. 



Fir' 10~^- f "^ or description see next Number. 



Fig. 11. — A. Robinaldina (?), D'Orb. From a specimen in the author's cabinet. 



Atherfield. 

 Fig. 12.— A. Robinaldina (?), D'Orb. From au unusually developed specimen in the 



Geol. Museum, Jermyn Street. 

 Fig. 13.— A. Hupiniana, D'Orb. From a specimen in the Geol. Museum, Jermyn 



Street. Sandown. 



{To be concluded in our next Number.) 



III. — Contributions to the Study of Volcanos. 1 



By J. W. Judd, F.G.S. 



The Ponza Islands. 



F the line passing through those three grand centres of volcanic 

 action — Vultur, Vesuvius, and Eporneo — be produced to the west- 

 ward, it will strike the very interesting igneous masses of the Ponza 

 Islands. These insignificant islands, which, from the early Eoman 

 times down to the present day, have figured in history only as places 

 of banishment for criminals, possess for the geologist the very highest 

 interest. This is due not only to the wonderful characters of the 

 rock-masses which compose them, but also to the admirable manner 

 in which these are exposed to our study by the extreme denudation 

 to which they have been subjected. 



In 1785 Sir William Hamilton visited these islands, and, being 

 greatly struck by the remarkable features which they present, not 

 only gave a short account of them in the " Philosophical Trans- 

 actions," but wrote to Dolomieu, calling his attention to the im- 

 portance of making a fuller examination of them. The illustrious 

 French philosopher spent some time in them during the following 

 year, and as the result of his studies his " Memoire sur les lies 

 Ponces" was published in 1788. In the year 1822 Mr. Poulett 

 Scrope made that careful survey of the whole of the islands, which 

 enabled him to lay before the Geological Society in 1827 his well- 

 known memoir upon them, 2 in which so many points of the highest 

 interest in connexion with the characters of the igneous rocks are 

 for the first time discussed. Lastly, in those very valuable investi- 

 gations concerning the microscopic structure of rocks and minerals, 

 which laid the foundation of a new and important branch of geological 

 science, Mr. Sorby in 1858 largely employed the very remarkable 

 rocks of Ponza, which the researches of Dolomieu and Scrope had 

 shown to present such interesting characters. 



After the detailed description of the Ponza Islands, accompanied 

 by elaborate maps and sections, contained in Mr. Scrope's paper, 

 the accuracy of which I have had the opportunity of verifying, 

 anything like a general memoir upon them would at the present time 

 be quite unnecessary. There are, however, certain features presented 

 by the rock-masses of Ponza which appear to throw important light 

 upon some of the at present " open questions " of geology. These it 

 may be desirable to call attention to in the present sketch. 



1 Continued from page 257. 2 Geol. Trans, ser. ii. yol. ii. 



