Weathering of Stone Mountain Granite

The weathering of Stone Mountain Granite (adamellitc) forms kaolinite, endcllite, allophane and gibbsite of which kaolinite is the most stable. Bulk density ranges from 2.65 in fresh rock to a minimum of 1.48 in saprolite. It is a good index of weathering. Abrasion pH ranges from 5.0 in saprolite to 9.3 in fresh rock, and is direct]y related to bulk density and the amount of clay mineral. Among the original minerals, biotite is least stable, followed by oligoclase. Residual mieroeline fragments occur at the base of the B horizon indicating that i~ is not completely weathered. Muscovite weathers slightly and quartz appears quite stable. Evidence shows that solution and reeonstitution is necessary in the formation of some if not all clay minerals. This is particularly evident in the formation of endellite veins. Expansion of the rock in the early phases of weathering occurs hut does not produce a significant increase in volume. I N T R O D U C T I O N Stone Mounta in is s i tuated 13 miles east of At lan ta , Georgia in the center of the Piedmont . The U.S. Geological Survey, Stone Mounta in Quadrangle map (1956) shows local relief to be about 100 ft, with an elevation of 900 ft above sea level. The m o u n t a i n itself is a rounded elongate monadnoek rising 800 ft above the surrounding area with an elevat ion of 1683 ft. The rock is an admell i te or quartz monzoni te (Herrmann, 1958, p. 29). The average composit ion of the rock in the area studied is given in Table 1. D a t a used in Table 1 are from H e r m a n (1958, p. 31) and Gran t (1962, p. 8). The climate is warm and moist, modified somewhat by the alt i tude. The average annua l precipi ta t ion is 47.58". However, var ia t ions in rainfall are considerable. The average J u l y tempera ture is 78.5~ (26~ and for J a n u a r y is 44.0~ (6.6~ (Mindling, 1941). The average p i t of ra inwater from 55 storms t aken in all seasons of the year is 5.0. The average tempera ture of the water is 15~