Abstract：The clay minerals and major elements of two basaltic weathering crusts and a tuffaceous crust in the Weizhou Island and the Xieyang Island were identified and measured by means of X-ray diffraction and X-ray fluorescence analysis. The results show that 7×10-10 m halloysite is the main constitutent in the clay minerals of every weathering profile, and goethite and hematite are also extensively existent. This suggests that these weathering crusts are in a fairly high degree of weathering. The abnormal variations of chemical indicators in surface weathering profiles and the existence of secondary calcite in one of the profiles imply the effect of compound alkalization on the weathering crust. Underground water has also made these profiles considerably different from the normally-developed weathering crust. Tropical monsoon climate has remarkably controlled the development of weathering crusts, but the specific compound alkalization of the island and the underground water have also exerted their influence on the development of the weathering crusts.