The intact section of the acient weathering crust in Sichuan Basin can be delimited inW three strata, i.e, those of elastic clay (the 3rd stratum)，the elastic brccia-clastic clay (the 2nd stratum) and the elastic breccia (the 1st stratum).The silt content of the ancient weathering crust is lower by about 15%, the fine clay content higher by more than 80% and the plastic index higher by 2.44-3.02% than those of the purplish soils derived from purple rock in the same region. The ancient weathering crust is relatively stronger in the accumulation of Si, Al, Fe and Ti and leathing of Ca, Mg, K and Na, as compared with its parent rocks underlying.