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larger and accreting material as they move around. They accomplish this either (a) by physical attachment of fine-grained material as they roll around, much in the manner of a snowball, and (b) by the chemical precipitation of material in solution, much as salt crystallizes from water during evaporation. In the former case they have thin concentric layers, and in the latter they have radiating sprays of crystals. However, a combination of both processes

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