Bentonites from the late Eocene Jackson Group (Cadell and Manning Formations) and Oligocene(?) Gueydan Group (Catahoula Formation) of Texas can be differentiated on the basis of X-ray diffraction and thin-section studies. Caddell and lower Manning bentonites consist of nearly pure well crystallized Ca (super ++) montmorillonite. Upper Manning and Catahoula bentonites consist of poorly crystallized montmorillonite, with varying amounts of quartz, cristobalite, feldspar, and volcanic shards. Bentonites from both groups in east and southwest Texas contain more other clay minerals, quartz, and feldspar than those of central Texas. Montmorillonite is generally well crystallized in bentonites formed in place, poorly crystallized in redeposited bentonites. Some Caddell and lower Manning bentonites in central Texas are thought to be lacustrine deposits.