In this paper, the writers dealed with the mineralogical properties of "clay concretion " which is contained in the strongly efflorescenced tuffuceous andesite in the area around Togō Lakes. Most of the clay concretions have a oval shape and consist of crust and contents. On the origin of such clay concretion shall be discussed in the following paper. The mineralogical properties were investigated by means of X-ray diffraction analysis, differential thermal analysis and electron micrographs. By these investigations it was recognized that the basal clay mineral (strongly efflorescenced tuffuceous andesite) in consisted almost with gibbsite and the protion of crust od concretion is consisted with gibbsite and halloysite hydrated. The inside crust of concretion is consisted almost with halloysitc hydrated. The inner content (dark reddish gray colour) is also consisted with halloysite hydrated but the degrees of crystallization and purity are lower than those of inside crust of concretion
