Abstract: A vitreous, perlitic rock near Etzatlán, west of Guadalajara, Mexico, has been argillized by hot spring activity. A continuous outcrop from fresh rock to relatively pure clay (halloysite) is exposed, and samples were collected along it to trace the progressive alteration. Alkali metals and silica were leached during argillation, as was shown by analyses of the solid samples and solutions from laboratory hydrolysis of the rock. The interpretation that the original clay mineral was endellite is supported by experiments treating the clay with K-acetate and ethylene glycol. The random arrangement of Si-O-Al in the glass is thought to have favored development of disordered stacking in the clay mineral formed, hence endellite, and random distribution of very fine-grained clay crystals in the mass of clay.