As altered volcanic material are being found in the Paleozoic, there is an interest in the criteria for their recognition. The relict glass structure, the nature and habit of the crystal grains, and the physical properties present evidence that indicates the mode of origin. Bentonite is a common type of altered volcanic material and may be recognized by the volcanic structure preserved in the clay substance, and the origin is confirmed by the presence of volcanic minerals. The chemical and physical properties of bentonite are variable and many are indefinite; thus, although they may contribute to its recognition, as a rule they can not be relied upon completely. Bentonites from several of the eastern United States, Nova Scotia, Arkansas, Texas, and New Mexico are describe in detail. It is concluded that many volcanic materials, even though highly altered, may be recognized by microscopic and physical studies and may contribute to the interpretation of the geologic history of a region.