This paper presents results of studies on the genesis of three soil profiles, which has been classified as series Guamium. This is an Ortho Dark Latosol which has developed from a siltstone of the Tubarão Group (Carboniferous period). Samples studied are from Piracicaba, SP. Statistical analysis of the sand fraction were performed, as well as, quantitative mineralogical analysis of the clay fraction. According to the sorting coefficient data, the two profiles (Pt and P2) are poorer sorting than the third one (P3). Both profiles, P1 and P2, have similar mineralogical features. The mean contents of kaolinite and gibbsite of Ap to R horizons, decreased from 87 to 50% and from 89% to zero, respectively, while the mica and allophane contents increased from 0 to 28% and from 1.2 to 22.5%, respectively. The profile P3 almost did not present any variation in the mineralogical analysis on depth. The kaolinite content presented an average of 74.8% and the others minerals presented the following averages: gibbsite 6.8%; mica 0.6% and allophane 16.9%. According to both mineralogical analysis of the clay fraction and statistical analysis of the sand fraction, the profiles P1 and P2 are similar and belong to the series Guamium, while the profile P3 have different features and does not fit in that series. The following weathering sequence were found from the mineralogical data of the profiles P1 and P2: Siltstone -> Kaolinite and Allophane -> Gibbsite.