Flipping through old National Geographics, I found the November 1965 'I Joined a Saharan Salt Caravan'. It describes the Tuaregs' trek from the oasis of Bilma, Niger, to the city of Agades, Niger. They carry sixty pound cones of a brown salt mined at Bilma by the Kinuri people.
I've been having fun trying all the different sorts of salt... does anyone know where I can get this Saharan salt... short of a West African vacation? (Someday, but not soon.)