Three hundred and forty-three samples, representing weathering profiles or sequences of some twenty-three different rock types collected in the field in Brazil, have been analyzed by wet-chemical, physical and x-ray diffraction methods for the purpose of understanding the processes involved in the decomposition of rocks by weathering and in the formation of clay minerals. Evidence indicates that there is a strong relationship between (a) the presence or absence of quartz in the parent rock and the type of decomposition product formed, and (b) the width of the 'transition zone' between fresh rock and completely weathered saprolite. Quartz-rich rock types exhibit wide, gradational weathered zones and usually form kaolinite or halloysite in well-drained, or perhaps smectite in poorly-drained, environments. Quartz-free rocks exhibit extremely narrow, sharply-defined weathered zones, and weather to gibbsite (plus or minus iron oxides) in well-drained, and smectite in poorly-drained, environments. Kaolinite found in the vicinity of quartz-free rocks is either formed by re-silication of gibbsite, or is of secondary origin (transported). Texture of the rock (aphanitic vs. phaneric) has little or no effect on weathering product formed, each producing identical materials.