Abstract: The weathering of biotite-plagioclase gneiss was studied in a warm humid climate under good drainage conditions, in De Kalb County, Georgia. A series of samples ranging from completely weathered to fresh rock were studied. The degree of weathering was determined by bulk specific gravity and abrasion pH of samples in various stages of decomposition. Quantitative determination of clay minerals was done by DTA. Results show that the stable products of andesine weathering are kaolinite and endellite, and that biotite weathers to a mixture of amesite and kaolinite. There is no volume change in the gneiss as it weathers from fresh rock with a bulk specific gravity of 2.78 to a completely friable saprolite with a bulk specific gravity of 1.4. The weathering is accompanied by an increase in kaolin minerals from zero to about 35 per cent. The process involves the solution of plagioclase and biotite with approximately simultaneous crystallization of clay minerals and hydrated oxides of aluminum and iron.