Specimen illustrations for a monograph on clay stones by J.M. Arms Sheldon, c1900. (Among the books published by American entomologist, educator, historian and museum curator Jennie Maria Arms Sheldon is "Concretions from the Champlain Clays of the Connecticut Valley" [1900]. A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, It is found in sedimentary rock or soil).
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