A formation known by borings made in Léopoldville and Brazzaville has recently been correlated with the Stanleyville beds (Upper Jurassic) by Egoroff and Lombard (1961). Fifty specimens of claystones belonging to this formation and to Cretaceous formations from the Kwango district have been analyzed by optical and x-ray methods. Two chemical analyses are presented. The claystones consist mostly of montmorillonite and quartz, with small amounts of potash feldspar, mica or illite, and kaolinite. Calcite, occasionally magnesian, and goethite are present in many Leopoldville samples. Siderite occurs uncommonly. Hematite is usually present in the samples from the Kwango district. Analcime, which is abundant in some mesozoic rocks from eastern and central Congo, has not been found. The origin of the various minerals is discussed.