Page:South African Geology - Schwarz - 1912.djvu/56

 Africa a similar ledge borders the land. It is at its widest south of Cape Agulhas, and is about 90 fathoms deep; beyond it the sea floor sinks rapidly to great depths. The great rollers from the Antarctic break on the edge of the submerged plateau, and this is therefore a dangerous place for shipping. It is known as the Agulhas Bank, but really it is the edge of a sunken portion of the continent.

The Solid Globe. — Beneath the atmosphere, and in part beneath the hydrosphere or watery envelope, is the crust of the earth, or lithosphere, as distinguished from the central portion or centrosphere. We surmise that beneath the ocean the crust is precisely similar to what we find on dry land, because we know the latter has often been covered by the ocean in past times, and it follows logically, therefore, that the oceans must as often have been dry land. Our investigation into the nature of the crust must necessarily be confined to the continents and islands. The whole arrangement of land and sea has continually changed in the past history of the globe, and what we now find in the contours is an accident of the present time which cannot have existed in the past, nor will be continued in the future. The crook on the east of South America fits so nicely into the Gulf of Guinea, and the Americas on their eastern border generally show lines which correspond with the contours of the western shores of Europe and Africa, that it has been suggested that the two were once united, but were riven asunder when the moon was supposed to have been whirled away from where the Pacific now is. When we find that at least twice, in comparatively recent times, Africa was united with South America, the impossibility of this view becomes apparent.