The minerals present in soils of the Taquaral Series are originated from diabase and glacial sedimentary rocks. The glacial contribution in materials to these soils reaches great depths. There is little variation in the percentage of these minerals in horizons A, B, and C, but they decrease in the lower layers, CDr horizon, where diabase is the most important contributor. The mechanism of diabase weathering was studied under conditions of high topography and good drainage mainly through the transformation of feldspars.