TRADE--METALS--MINES--VEGETABLE PRODUCTIONS--GUMS--OIL--COTTON--DYE-STUFFS. The trade of Africa for an almost indefinite time must consist of the materials for manufactures. The fact that old formations reposing on granite, or distorted by it, form a large proportion of its geological surface, indicates that useful metals will probably be found in abundance. In comparing it with California and Australia as to the probability of finding deposits of the more valuable metals, two circumstances of great importance must be kept in view. These countries were possessed by natives who had no domesticated animals, and therefore were not called upon to exercise over the soil the same inquisitive inspection for herbage and water as were required from the races among the mountains and deserts of Africa, so that the chances of finding any thing were not the same. The other circumstance is, that metals were comparatively little known to the aborigines of California, and not at all to those of New Holland, so that discoveries of the kind would neither be sought for, nor reckoned of much value when they occurred. On the other hand, metals of all kinds have during indefinite eras been regarded as of high importance, and have been used in various ways by the African nations. Copper, and some alloys of it, seem to be used for ornaments throughout the whole south. These are smelted from the ores by the natives. They also manufacture their own iron. Their desires, therefore, and their necessities, and their arts, render it probable that no deposits of metals exist, except such as require scientific skill to discover, and mechanical resources to procure. Gold is not in this predicament. Wherever it occurs in abundance, it has been collected by elemental waste from disintegrated rocks, and is mixed with gravel and alluvial matters in those portions where men of nomadic habits, and familiar with metal ornaments, would most readily meet and appropriate it. Some, probably a great proportion, of the gold of ancient Egypt, was got by a laborious process of grinding, on which their wretched captives were employed. This would not have been the case if the metal had been found plentifully throughout the extensive regions with which they were acquainted. An addition to the metallic riches of the world from Africa, is therefore to be looked for in the discovery of deep-seated mines, if there are any, and in better modes of working those which exist, particularly the alluvial deposits of gold along the northern shores of the Gulf of Guinea and the shores of the Mozambique Channel. The present export of gold from all Africa, probably amounts to about two millions of dollars per annum.