Of geographical necessity the external culture contacts of Africa have been mainly by sea, and consequently in order to explain the areal differentiations apparent in the cultural history of the continent it would seem necessary to supplement research on the landward traditions of African societies by investigations into the cultural patterns of sea regions embracing the oceanic faces of the continent. In the macro-regional structure of Africa it is possible to recognize entities such as Mediterranean Africa, Atlantic Africa, and Indian Ocean Africa, which possess distinctive personalities that cannot be entirely understood by landward reference but find their true provenance in the cultural dynamics of wider maritime theatres of action. Thus many of the keys to the cultural history and character of the eastern face of Africa must be sought not in Africa itself but in the changing patterns of the Indian Ocean region of which this African zone forms an integral part. As a student of the historical geography of the Indian Ocean, I am concerned here with but one environmental element in the structure of this region and its significance to some aspects of pre-colonial African history.