/. W. Judd—On Volcanos. 307 



place under enormous pressure; and we never find it assuming the 

 vitreous structure. In the rock of Ponza the pressure was evidently 

 far less, and the rock occasionally passes into a more or less glassy 

 form ; while in the lavas known as Liparites, where all superin- 

 cumbent pressure is got rid of by their extrusion at the surface, the 

 tendency to pass into the vitreous condition is, as we have seen, 

 extreme. By the study of different portions of igneous masses we 

 are able, therefore, to trace every stage in the transition from the 

 most typical granite to the most perfect glass and pumice. 



The relation of the glassy portions of the rock of Ponza to the 

 ordinary crystalline varieties are, as pointed out by Mr. Scrope, 

 worthy of the most careful study. In almost every case the dykes 

 or intrusive sheets of crystalline rock are at their planes of junction 

 converted for a greater or less thickness into a glassy material. 

 Three different causes suggest themselves as possibly tending 

 towards this result. 



(1). The more rapid cooling of the liquefied masses on their outer 

 surfaces. 



(2). The enormous friction, of which we have the clearest evi- 

 dence, between the intruded matter and the agglomerates through 

 which they were forced. This might operate in two ways : by 

 crushing up the solidifying particles, and rendering them easy of re- 

 fusion ; and by the actual development of additional heat from the 

 friction. The probability of this kind of action having gone on 

 is shown by the fact that not only are the dykes of solid rock con- ' 

 verted into glass at their sides, but the masses of agglomerate them- 

 selves, near the lines of junction, also pass into obsidian. 



(3). The smaller amount of resistance offered by the agglomerates 

 to the expansion, which, as we have seen, takes place in the passage 

 from the crystalline to the colloid state, would favour the production 

 of obsidian on the outer surfaces of the intrusive masses. 



It may well be conceived how, with the presence of such con- 

 ditions as we have indicated, the most remarkable transitions of rock 

 structure from the glassy to the crystalline may be produced ; 

 accompanied by the development of the most singular examples of 

 brecciated, ribboned, and contorted appearances. 



There are a number of other interesting features which have been 

 already described as being exhibited by the Ponza rocks, to which 

 want of space will prevent us from doing more than making the 

 barest allusion in this sketch. Such are the interesting prismatic 

 forms assumed by them on the smallest as well as on the largest scale ; 

 the remarkable globiform concretions in some of their vitreous masses; 

 the changes undergone by them in consequence of the passage of water 

 and acid gases through them; and the formation of crusts of carbonate 

 of lime on their surfaces, and of calcareous sandstones in their 

 hollows, through the agency of land-shells. For details on these 

 subjects I must again refer to Mr. Scrope's memoir. The causes of 

 the production of the banded structure in these rocks I shall have 

 occasion to discuss on a future occasion. 



In concluding this imperfect sketch of a district, which, among 



