Black Man's Burden by Mack Reynolds

Once the city of Timbuktu was more important in population, in commerce, in learning than the London, the Paris or the Rome of the time. It was the crossroads where African traffic, east and west, met African traffic, north and south; Timbuktu dominated all. In its commercial houses accumulated the wealth of Africa; in its universities and mosques the wisdom of Greece, Rome, Byzantium and the Near East -- at a time when such learning was being destroyed in Dark Ages beset Europe.

Timbuktu's day lasted but two or three hundred years at most. By the middle of the Twentieth Century it had deteriorated into what looked nothing so much as a New Mexico ghost town, built largely of adobe. Its palaces and markets has melted away to caricatures of their former selves, its universities were a memory of yesteryear, its population fallen off to a few thousands. Not until the Niger Projects, the dams and irrigation projects, of the latter part of the Twentieth Century did the city begin to regain a semblance of its old importance.