On the basis of the identification of the microstructures and mineral composition, the authors discovered strongly corrosive microstructure of primary quartz and the microgranular textures of secondary quartz and chalcedonite which do not exist in the north subtropics and areas farther to the north,and this discovery provides strong evidence for the determination of the change of the paleoenvironment. In the light of a synthetical analysis of the microstructure, sporo-pollen and chemical composition, it has been determined that the corrosive relic red earth, which consists dominantly of clay minerals was formed in the middle subtropics with a pluvial and hot climate and that the dominant vegetation was evergreen broadleaved forest. The soil type of the red earth corresponds to the recent red earth in the middle subtropis and its age is Miocene.