Formation of attapulgite is related to calcareous and siliceous chemical deposits. In a marine environment, deposition of phosphates intervenes. The attapulgite is authigenic in origin, as is also the montmorillonite in chalks. The elements necessary for sedimentation characterized by abundant Si, Ca, and Mg are derived from differential leaching of material in continental areas which are being subjected to laterization. Such a history can be reconstructed for the Gao basin in the Sudan, French West Africa.