Seismic wave refraction data in the rhyoritic (Arima Group) area, north of Osaka, were examined by mirage method. The linear relationship between the subsurface increasing rate k of seismic velocity to the surface velocity V₀ is so detected here as in granitic area. Characteristic obliqueness of the iso-velocity surface to the land surface has been derived from the dependence of surface velocities upon the ground slope I and upon the equivalent slope length l. Surface velocity and ground slope get larger following the distance from watershed, and this tendency is hold until the point of l=100m which is the boundary of ridge part to river channel viewed from l-I relationship in the study area. Topographic difference between rhyoritic area and granitic one is shown clearly by this critical distance.
The weathering process near land surface is described approximately by diffusion equation with surface evaporation boundary condition, and this is proved by the proportionality of Laplacian on the density ratio, C=ρ/ρ_r, to the weathering rate deduced from erosional one, where ρ is apparent density of weathered rock and ρ_r that of fresh one. Conclusively, the diffusivity constant for rock weathering is estimated to have the order of magnitude of 0.1mm²/year for rhyoritic rock in this area, based on the rate of erosion in the neighbouring Rokko area by slope failures. The value is almost one order smaller than that for granitic rocks, though detailed check of the specific data concerned is the future problem.