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Alpha particles have been measured coming from the surfaces of rocks covered with dark red-brown rock varnish. as well as rocks that appear to have little. if any. varnish. A pronounced peak at 5.3 MeV indicates the presence of Po-210, a short-lived natural-radioactive element. Surface activities for 33 samples range from 0.008 Bq/cm(2) to 0.065 Bq/cm(2). It is estimated that this nuclide is concentrated 10(11)times in these paper-thin coatings above its concentration in ground-level air. Gamma rays from the decay of Cs-137, a product of testing nuclear weapons some 50 years ago, were also detected. Analysis of samples of varnish stripped from the rock revealed traces of Pu-239.240 and Pu-238. The presence of all of these isotopes strongly supports the theory that varnish films derive their building blocks from the atmosphere and, with time, all rocks in arid environments will become coated. (C) 2004 Elsevier Ltd. All rights reserved.