Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/181

Rh The surface of the earth also offers numerous other indications that the essence of vulcanicity is to be sought in a passive extrusion of the inclusions of sima from the sial crust. This is best shown by the island festoons. On account of the curvature, compression must occur on the concave inner side; and tension on the convex outer margin. Actually, the geological structure is strikingly uniform: the inner side always bears a chain of volcanoes; the outer shows no vulcanicity, but strong fissures and faults. This universally recurring arrangement of volcanoes is so remarkable that it appears to me to be of the greatest importance for the question as to the nature of vulcanicity. “A volcanic inner zone and two outer zones can be distinguished in the Antilles, of which the outermost is built of the most recent deposits and has a lower altitude (Suess). The contrast of a highly volcanic inner zone and an outer zone with restricted vulcanicity also occurs in the Moluccas (Brouwer) and in Oceania (Arldt). The analogy with the arrangement of volcanic zones on the inner side of thrust zones, as in the Carpathian or Variscan hinterland, is obvious.” The position of Vesuvius, Etna, and Stromboli fits in with this idea; and of the islands of the arc of the South Antilles between Tierra del Fuego and Graham Land, it is just the strongly curved central ridge of the South Sandwich Islands that is basaltic, and one of its volcanoes is still active. Brouwer describes a particularly