Concretionary Structure

Concretionary Structure is a condition in rocks produced by molecular aggregation subsequent to the deposition of the strata, whereby the material of the rock is formed into spherules or balls, as in the concretions of magnesian limestone and the somewhat similar structures occasionally seen in certain tuffs and crystalline igneous rocks. Concretions are nodules, balls, or irregular masses of various kinds which occur scattered through the body of a rock, and consist of mineral matter which was formerly diffused through the material of the rock. Some of these concretions are crystalline, as gypsum in clay; others may be spherical, and have an internal radiating structure, as iron-pyrite in shale. Fantastically shaped concretions are not uncommon in certain fine clays; such are the 'fairy-stones' of the country people here, and the 'lösspuppen' of Germany.