Page:A Treatise on Geology, volume 2.djvu/245

 CHAP. IX. or coralligenous growth. Grenada, St. Vincent, St. Lucia, Dominica, Montserrat, Nevis, St. Christopher, St. Eustachia, are entirely volcanic; Martinique, Guadaloupe, Antigua, St. Bartholomew, St. Martin, St. Thomas, are partially volcanic, and partially calcareous. The line of these volcanic islands forms an arch convex to the eastward.

On the continent of Africa, the notices of volcanic districts are slight and incomplete. Perhaps between the Nile and the Red Sea, as Ruppell and Jomard state, volcanic action is not extinct. In Mount Atlas hasaltic eruptions appear. The African islands, on the contrary, are nearly all, almost exclusively, volcanic. From the Azores, which are usually reckoned as European, the Madeira Isles continue the Atlantic system of volcanic action to the group of the Canaries. Further south, the Cape de Verde Isles, Ascension, Fernando Po, Prince's Island, St. Helena, Tristan d'Acunha, Gough's Island, are so many points of active or extinct volcanic fire. Madagascar, Bourbon, and Mauritius contain abundantly the effects of the same cause.

The circumstances observed in these various groups differ extremely. In Madeira and Porto Santo, Ascension, St. Helena, Tristan d'Acunha, the volcanic fires are extinct, and their effect has generally been to up-heave stratified rocks covered by volcanic accumulations. The Canary group has, in Lanzerote, a low volcanic tract liable to burst suddenly after long intervals (from 1736 to 1834.), and a vent immensely elevated for the escape from gaseous emanations and explosions, in the Peak of Teneriffe, which rises to between 11,000 and 12,000 feet, out of a concentric base of basaltic jocks, between 3000 and 4000 feet high. Von Buch believes this fact to be in favour of his general doctrine of craters of elevation, which is also supported by him upon the evidence of the form of Palma, another of these islands, which have all (according to this view) been raised from