Atmospheric agent（weathering）may transform "plutonic coarse crystalline rocks" into a poorly coherent mass, selective weathering may alter its constituent plagioclase and biotite crystal into clay minerals.
In order to clarify the weathering processes, some studies was carried on boring core sarmples of three kind of plutonic rocks, quartz diorite, granodiorite and granite from San-in district.
All boring core samples were investigated by means of standard penetration test, measuring of abrasion pH and bulk density, chemical analysis and X-ray powder method.
The present paper descibes the results of detailed studies on these weathered boring core samples, especially the mutual relation between relative amount of clay mineraes, variation of chemical composition of biotite with weathering processes, alteration of plagioclse, abrasion pH and N-value from standard penetration test.