300 



J. W. Judd — On Volcanos. 



Twenty miles W.N. W. of the first of these old submerged volcanic 

 cones is situated the other and principal group of the Ponza Islands, con- 

 sisting of Ponza, Palmarola,and Zannone, with many smaller islets and 

 rocks. The highest part of this group of islands, which are evidently 

 the more prominent points of another submerged tract, is the mountain 

 mass forming the southern part of the island of Ponza, and known as 

 the Monte della Guardia, which rises to the height of 95 1 feet. This 

 consists of a bulky bed of ordinary trachytic lava, resting upon strati- 

 fied tuffs, both precisely similar in character to those of Ventotiene and 

 Ischia. The form assumed by this mass of lavas and tuffs clearly 

 indicates that it is the sole remaining fragment of another volcano, 

 composed of the same materials as those to the eastward. (See Fig. 18.) 



Fig. i8. — The Headland Monte della Guardia in Ponza. 



a, Columnar trachyte, b, Stratified tuffs, c, Pumiceous agglomerates, d, Intrusive 

 masses of Quartz-trachyte. 



In the case of the island of Ponza, however, this relic of an old 

 volcano is seen to rest unconformably upon a still older series of 

 rocks, which constitutes by far the larger portion of the entire 

 group of the Ponzas. These rocks, although evidently of igneous 

 origin, like those which rest upon them, nevertheless offer, alike 

 in their chemical and mineralogical . constitution and in their 

 geological relations, a most remarkable contrast to the latter. While 

 the overlying, and evidently newer, rocks are composed of ordinary 

 sanidine-trachytes, with interbedded stratified tuffs, clearly the re- 

 sult of volcanic action at the surface, the latter are made up of highly 

 siliceous pumiceous agglomerates, through the midst of which dyke- 

 like masses of a rock of the same composition as granite, and 

 approaching that rock in many of its characters, has been forced. 

 (See Fig. 19.) 



The remarkable features assumed by these older rocks of Ponza, 

 as the result of the mechanical strains to which they have been sub- 

 jected during their consolidation and crystallization, powerfully 

 arrested, as we have seen, the attention of those pioneers in the 

 study of Vulcanology, Hamilton, Dolomieu, and Scrope ; and these 

 rocks are still worthy of the most diligent and attentive study, both 

 as regards their physical relations and their minute structure, by all 



