The weathering of several basic igneous rocks of Lower Carboniferous age has been studied by optical microscopy and X-ray techniques. Although similar in mineralogy, the rocks have different textures and the fine-grained ones are most resistant to physical disintegration. In every rock, olivine has been affected by hydrothermal alteration to produce layer lattice silicates. The weathering of these products and the other mineral constituents of the rocks is described.

The fine-sand fraction (0·2-0·02 mm.) was selected for the purpose of determining relative stabilities of the primary minerals under weathering conditions. In order of increasing stability, the arrangement is labradorite, augite, magnetite, ilmenite, and hematite. The silt (0·02-0·002 mm.) and clay (< 1.4 μ) fractions contain saponite and/or vermiculite depending mainly on the weathering conditions of augite and felspar. Kaolin is present in both size fractions but illite is scarce. The cation-exchange capacity (c.e.c.) of both silt and clay fractions is consistent in most cases with the layer lattice silicate contents.