Ch. XXIX.] STRUCTURE OF VOLCANIC ROCKS. 



Fig. 690. 



619 



^^Vbs&Ou^ 



^glWN^S 3 



Basaltic pillars of the Kiisegrotte, Bertrich-Baden, halfway between Treves and Coblentz. 

 ^Height of grotto, from 7 to 8 feet. 



rocks the globular structure is so couspicuous that the rock has the 

 appearance of a heap of large cannon balls. According to the theory 

 of M. Delesse, the centre of each spheroid has been a centre of crys- 

 tallization, around which the different minerals of the rock arranged 

 themselves symmetrically during the process of cooling. But it was 

 also, he says, a centre of contraction, produced by the same cooling. 

 The globular form, therefore, of such spheroids* is the combined result 

 of crystallization and contraction.* 



A striking example of this structure 

 occurs in a resinous trachyte or pitch- 

 stone-porphyry in one of the Ponza 

 islands, which rise .from the Mediter- 

 ranean, off the coast of Terracina and 

 Gaeta. The globes vary from a few 

 inches to three feet in diameter, and are 

 of an ellipsoidal form (see fig. 691). The 

 whole rock is in a state of decomposition, 

 " and when the balls," says Mr. Scrope, 

 " have been exposed a short time to the 

 weather, they scale off at a touch into 

 numerous concentric coats, like those of 

 a bulbous root, inclosing a compact nu- 

 cleus. The laminee of this nucleus have 

 not been so much loosened by decompo- 

 sition ; but the application of a ruder 

 blow will produce a still further exfolia- 

 tion." f 



Globiform pitch stone. 

 Luna, Isle of Ponza. 



Chiaja di 

 (Scrope.) 



* Delesse, sur les Roches Globuleuses, Mem. de la Soc. Geo-1. de France, 2 s6r. 

 torn. iv. 



f Scrope, Geol. Trans., Second Series, vol. ii. p. 205. 



