J. W. Judd — On Volcanos. 



301 



who desire to investigate the nature, mode of action, and products of 

 volcanic forces. 



Fig. 19. — Western Spur of Monte della Guardia. as seen from the North side 



of Luna Bay. 



a, Trachytic lava, i, Stratified tuffs, c, Intrusive masses of quartz-trachyte with their 

 edges passing into obsidian porphyry, d, Pumiceous agglomerates. 



The great masses of pumiceous agglomerates, traversed by dykes 

 and sheets of the peculiar quartz- trachyte which together constitute 

 the greater part of Ponza, and the whole of Palmarola, are in Zan- 

 none seen in contact with sedimentary rocks of Cretaceous age 

 (Hippurite limestones) resembling those of the nearest point of the 

 mainland, Monte Circello. To the student of the older volcanic 

 rocks those features of local metamorphism presented by the lime- 

 stones of Zannone, which were first pointed out by Mr. Scrope in 

 1827, cannot fail to be of the highest interest. On the north-east 

 side of the island the Cretaceous limestones exhibit precisely the 

 same characters as at Monte Circello ; but as we approach the igneous 

 masses extruded through them, they are found, becoming highly 

 crystalline and by degrees passing into a dolomite. A specimen of 

 this altered rock, which my friend Professor Guiscardi, of the Naples 

 University, examined for me, was found to exhibit little or no effer- 

 vescence upon the application of acid to it ; but when powdered and 

 heated with the acid, carbonic acid gas was at once disengaged. 



But not only has the limestone undergone considerable changes 

 near its junction with the igneous rocks, but these latter have also 

 themselves been greatly affected, passing into a compact highly 

 siliceous material, with a strikingly conchoidal fracture. It is inte- 

 resting to notice that the intrusive rocks of similar composition in 

 the Hebrides have undergone precisely similar changes near their 

 contact with stratified masses. 



We have thus evidence that in the Ponza Islands great eruptions 

 of igneous rocks of the most highly acid class have taken place 

 subseqiiently to the deposition of the Cretaceous rocks, and that after 

 these earlier volcanic masses had suffered greatly from denudation, 

 which appears to have removed all the cones and lava-streams, 

 leaving only masses of agglomerates traversed by dykes and sheets 

 intruded among them, a second series of volcanic outbursts took 



