Abstract: Hydrated halloysite clays altered from glassy tuff during weathering were examined in detail by x-ray, thermal, and electron micrographic methods. The electron micrographs show fibrous crystals of hydrated halloysite associated with many rounded grains, the former frequently jutting out from the latter. X-ray powder diffraction data and thermal curves show clearly the presence of hydrated halloysite, and on closer examination dehydrated halloysite and a small amount of montmorillonite can be detected. The presence of allophane as an accessory mineral is suggested in certain specimens. Furthermore the degree of crystallinity of hydrated halloysite varies slightly from specimen to specimen. The variation of the degree of crystallinity of the hydrated halloysite and of the kinds and amounts of accessory clay minerals, and the variation of the shapes of the particles observed in the electron micrographs are generally related to each other. Hydrated halloysite of a relatively well-crystallized sort generally co-exists with halloysite (dehydrated) and consists of elongated crystals having sharp edges; rounded grains are composed entirely of these fibrous crystals. On the other hand, a poorly crystallized sort tends to be associated with allophane and to show extremely fine hairlike, twisted fibers; these are associated with well-defined rounded grains. A genetic process of clay-mineral formation from volcanic glass is considered.