Low amounts of selenium occur in granites, rhyolites, rhyolitic pumices, limestones, and schists. High amounts occur in andesites (particularly ashes), Upper Cretaceous argillites, and basalts. The selenium content of soil-forming rocks averages 0-42 p.p.m. Se; this value is strongly influenced by the large areas covered by sedimentary rocks having a clay component and it is higher than an average based upon igneous rocks. Weathering processes at the soil surface result in a retention of selenium and prevent a recycling of the element via the marine environment.