Mineralogical investigation of lithosols formed on dolomite in a semiarid, subalpine environment reveals up to 30 percent contamination of the very fine sand fraction by rhyolitic ash from a distant source and as much as 50 percent local contamination of the silt fraction, largely attributable to aeolian processes. Total extraneous particles range from 3 to 73 percent in sand or silt fractions of a given soil sample. Short-term dust-trap and snow-residue collections place minimum aeolian deposition at approximately 0.00014 grams sq cm−1 yr−1, a rate sufficient to account for the amounts of observed contamination. The data emphasize the need for precaution in interpretation of soil mineralogy and chemistry, under many climatic and topographic influences and in areas subject to Pleistocene volcanism.

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