1. Ajayi, J. F. A.: Population Census of Nigeria: List of Historical Events for Determination of Individual Ages, Ibadan 1962.
2. Barr, Lorna: NAWAPA: A Continental Water Development Scheme for North America? Geography,60, 111–119 (1975).
3. Bovill, E.: Enchroachment of the Sahara on the Sudan, J. of African Society,20, 174–185 and 259–269 (1921).
4. Bryson, R. A.: Climatic Modification by Air Pollution, II: The Sahelian Effect. Report No. 9, Inst. Environmental Studies, University of Wisconsin (1973).
5. David, N.: Extensive Development of the Agricultural Sector in the Semi-arid and Northern Savanna Zones of West Africa; in Dalby and Harrison Church R. J. (eds.): Drought in Africa, S.O.A.S., University of London, 1973.