Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 ^^Vbs&Ou^ ^glWN^S3 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 arran
Elements of geology, or, The Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments elementsofgeolog00lyel Year: 1868 ^^Vbs&Ou^ ^glWN^S3 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
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