Possibly, these combined causes of aridity and heat would have proved insufficient to convert such large portions of the African plains into a dreary waste, had not some convulsion of nature—as for instance the irruption of the ocean—on some occasion deprived these flat regions of their nutrient soil, as well as of the vegetation which it supported. The epoch when this occurred, and the nature of the forces which determined the irruption, are alike shrouded in the obscurity of the past. Perhaps it may have been the result of the great rotatory current[24], which drives the warmer waters of the Gulf of Mexico over the bank of Newfoundland to the old continent, and by which the cocoa-nut of the West Indies and other tropical fruits have been borne to the shores of Ireland and Norway. One branch of this oceanic current, after it leaves the Azores, has still, at the present time, a south-easterly course, striking the low range of the sandy coasts of Africa with a force that is frequently fraught with danger to the mariner. All sea-coasts—but I refer here more particularly to the Peruvian shore between Amotape and Coquimbo—afford evidence of the hundreds, or even thousands of years, which must pass before the moving sand can yield a firm basis for the roots of herbaceous plants, in those hot and rainless regions where neither Lecideæ nor other lichens can grow[25]. These considerations suffice to explain why, notwithstanding their external similarity of form, the continents of Africa and South America present the most widely different climatic relations and characters of vegetation. Although the South American Steppe is covered with a thin crust of fruitful earth, is periodically refreshed by rains, and adorned with luxuriant herbage, its attractions were not sufficient to induce the neighbouring nations to exchange the beautiful mountain valleys of Caracas, the sea-girt districts, and the richly watered plains of the Orinoco, for this treeless and springless desert. Hence on the arrival of the first European and African settlers, the Steppe was found to be almost without inhabitants.