Mr. PouLETT ScROPE 071 the Geologi/ of the Ponza Isles. 206 



cong-lomerate is found almost without a single exception to be altered at the 

 plane of contact with the prismatic trachyte. 



This alteration extends from two to thirty feet in depth when the natural 

 section of the rock cuts the plane of junction at right angles. When its direc- 

 tion is oblique or partially parallel to that plane, of course the alteration ex- 

 tends over a much larger surface of the bare cliff, and at first induces the false 

 notion of the existence of very considerable masses of the altered rock. This 

 consideration must be borne in mind in studying the profiles of these cliffs. 



The change consists in the gradual transition of the white and earthy base 

 of the conglomerate, as well as of the cellular and glassy fragments it incloses, 

 into a compact resinous trachyte or pitchstone porphyry, of a dark bottle- 

 green colour, containing numerous crystals of glassy felspar and bronzed 

 mica, and breaking readily into lamellar and rhomboidal fragments. This at 

 least is its predominant character : occasionally it exhibits a testaceo-globular 

 structure, becoming a pearlstone similar to that of Iceland and the Andes, of a 

 greenish-gray colour and pearly lustre. In other places it assumes a still more 

 regular globular concretionary division both on the large and small scale. In 

 the latter case the rock resembles an aggregation of beads, breaking into con- 

 centric layers ; this variety contains cellular and fibrous parts like the vesicular 

 obsidian of Lipari. The large globiform structure is strikingly exemplified 

 in a rock which projects from the cliff within the Chiaja di Luna, near the 

 entrance of the Roman gallery *. The alteration appears here to have pro- 

 ceeded to a great depth ; and the globiform structure occurs in that part of 

 the pitchstone bed which is nearest to the junction with the prismatic tra- 

 chyte. The rock shows a tendency to the columnar division, the columns 

 separating into large globes or ellipsoids, placed one above the other. These 

 balls, when they have been exposed a short time to the weather, desquamate 

 at a touch into numerous concentric coats, like those of a bulbous root, in- 

 closing a compact nucleus, of which the laminae have not been sufficiently 

 loosened by decomposition ; though the application of a ruder blow will pro- 

 duce a still further exfoliation. The globes vary from a few inches to three 

 feet in diameter. This structure passes not only into the columnar, but also 

 into the lamellar ; the lamellae being either straight or twisted into the most 

 intricate contortion. The longest axis of the ellipsoids is invariably in the 

 direction of the lamellar or prismatic system to which they belong. 



These varieties of natural division are certainly not produced by decompo- 

 sition, which has evidently only assisted in disclosing an original configuration. 



* See Plate XXIV. fig. 4. 



