Only recently have historians devoted much attention to monetary developments in African history, primarily because the substantivist school of economic anthropology, which has argued that so-called western economic theory does not apply to African situations, has dominated the field. This view has been increasingly under attack in recent years, particularly by a new group of economic historians who have found many aspects of formal economic theory useful in the reconstruction of Africa's past. Marion Johnson's pioneering work on the gold mithqal and cowrie shell, for example, has documented the spread of a common currency over much of West Africa, throughout an area encompassed by Lake Chad in the east, the upper reaches of the Senegambia in the west, the southern Sahara in the north, and the region between the Volta basin and the Niger Delta in the south. The study of other currencies, including the copper rod standard of the Cross River basin in Nigeria and Cameroons, and the cloth money of the Senegambia, has demonstrated the importance of other standards besides cowries and gold, so that it is now known that virtually all of precolonial West Africa had economies sufficiently developed to require the use of circulating mediums of exchange and units of account. This breakthrough raises a number of important questions which seriously challenge, if not completely undermine, the predominant view that Africa's past, down to very recent times, has been subsistence oriented, non-market directed, and basically static.
Comparison of seismic processing and interpretation tradeoffs between Kirchoff and finite-difference migrations, using poststack migrated data in the west Niger Delta, Nigeria