Page:The origin of continents and oceans - Wegener, tr. Skerl - 1924.djvu/83

Rh Its jagged outline stands in sharp contrast to the practically straight course of the margin of the South American shelf, and indicates a special origin. We are probably dealing in this case with molten sialic masses (granite) from the under side of the South American block, which as a result of its displacement have emerged on its posterior edge. In a similar manner the granite masses of the Seychelles have appeared from under the margin of Madagascar or India, and, to anticipate, probably also the substructure of Iceland.

The deltaic projection of the African coast at the estuary of the Niger need not be quite omitted from the reconstruction, for the north-west of Brazil shows a corresponding but too small embayment. This projection must, however, be greatly reduced in order that the blocks may be brought into contact. It has been emphasized by several authors that not merely deltaic deposits exist there. The assumption appears to me very probable that the projection—or at least a part thereof—is a plastic deformation of the African block which we can compare with a squeezing-out. Such a process could easily have taken place in the angle between the two great lobes of North-east and South Africa. We shall learn later of another very similar process in the remarkable triangle of land in the region of the Red Sea between Abyssinia and the Somaliland peninsula. The vulcanicity along the line of fracture which traverses the Cameroons, bears the volcanic Cameroon mountain and is continued in the volcanic islands of Fernando Po, Prince’s Island, St. Thomas, and Anno Bom, may be connected with these manifestations of pressure. The phenomenon will be repeatedly met with, that volcanoes occur where the horizontal movements of the earth’s crust produce a pressure which forces the mobile inclusion of sima out of the sial blocks.