Abstract
In the San-in area some kinds of granites belonging to the Paleogene period are widely developed. The writers have investivated the joint clay of these granites connecting it with the mechanism of the collapsed caused by heavy rain fall. Main types of joint clay assemblage are as follows: (1) monmorillonite type, (2) sericite type, (3) monmorillonite-sericite assemblage type, (4) monmorillonite-sericite-kaolinite assemblage type, (5) monmorillonite-sericite-hydrated halloysite assemblage type, and (6) sericite-kaolinite-hydrated halloysite assemblage type. The characteristic form of slope failure on weathered granites caused by heavy rain fall is landside that weathered rock blocks enclosed by joint planes slither down joint plane. In the event of such a collapse, these joint clay acts as cleavable plane and lubricant.
