The weathering sequency of clay minerals is usually regarded as one of the important indexes for the classification of genetic soil groups. After a detailed review on literatures concerning tropic soils it is found that a great deal of confusion has been made in this respect. Actually some red soils are derived from highly weathered materials of earlier geological ages, and they inherit their clay minerals largely from old ferruginous crust. Others are young residual soils derived directly from various parent rocks. Composition of clay minerals of the latter soils reflects an incipient stage of present soil forming process. Soils having great diversity of SiO₂/Al₂O₃ and SiO₂/R₂O₃ ratios can be found in a restricted area in tropic and subtropic regions. Such divergent soil types were found in Kunming district, Yt}nnan through a detailed soil survey in 1958-1962.