Soil genesis on felsic rocks in the St. Francois mountains. I. The role ofsaprolite and its influence on soil properties.

One soil profile from Taum Sank Mountain and two soil profiles from Mudlick Mountain in the St. Francois Mountains of Southeast Missouri were investig ated to determine if the soils evolved from the underlying rock unit and to characterize the role of saprolite on soil genesis. Taum Sauk Mountain is composed of rhyolite lava and ash now of the Van East Group. The rock is qu artz and orthoclase phenocrysts in an orthoclase groundmass. Mudlick Mounta in consists of rock composed of orthoclase, oligoclase, and a minor amount of quartz phenocrysts in a groundmass of quartz and orthoclase. The shallow Aquic Hapludult formed oil the summit of Taum Sauk Mountain shows no evide nce of saprolite, whereas the two Typic Paleudalfs formed on Mudlick Mounta in rest on a thick layer of saprolite. The presence of minerals in the host rock that weather more easily is largely responsible for the development o f the saprolite on Mudlick Mountain. The very strongly acid Typic pic Paleu dalfs have textures ranging front cobbly silt loam to cobbly sandy loam for the eluvial horizons, whereas the argillic and C horizons are generally fi ne or clayey-skeletal. The extremely acid Aquic Hapludults have a fine-silt y texture. The clay mineralogy is mixed, with hydroxy-Al interlayered vermi culite and hydrous mica prominent in the eluvial horizons and kaolinite dom inating the Bt and C horizons. The aluminum hydroxide potentials of soil so lutions extracted from selected horizons are saturated or slightly supersat urated with respect to kaolinite. Elemental analysis demonstrates that the soil and saprolite originate from the same parent material and that element al depletion is important to the formation of the saprolite. Iron oxides re present nearly all of the total Fe, and Fe is correlated highly with clay c ontent. The rare earth elements are more abundant in the soil than in the h ost rock. The weathered thin loess capping is suggested as the source of th e rare earth elements.