A concretion is an aggregate of inorganic matter in the shape, roughly, of a ball, disc, rod, or irregular nodular body. Usually the aggregation or accumulation started around a small center grain or particle and continued in the growth of layers about it like the shells of an onion, or in the growth of needle-like fibers which radiate from the center like pins stuck into a spherical pin cushion. Concretions vary in size from buck-shot (buck-shot concretions in the soil) to oddities ten or twenty feet in diameter, or even longer in elongate forms. The variety about one-sixteenth or one-thirty-second of an inch in diameter is called an oolite (pronounced oh-oh-lite). Some chert of southern Missouri, most of the diaspore and burley clay, and a limestone cropping out near Louisiana, Missouri, are made up partly to almost entirely of oolites. See page 29. Concretions may be composed of pyrite, calcite, limonite, chert, cemented sandstone, or even cemented clay. They are usually recognized by their structure after the previously enclosing rock has been eroded. Thus pyrite, limonite, or calcite (limestone) concretions remain after shale has softened and washed away, chert remains after limestone, and strongly cemented “irony” (limonite or hematite) or siliceous sandstone concretions may be found on the outcrop where the softer or less resistant host rock has been carried off. The irregular-shaped, intergrown, nodular limestone concretions (sometimes called “loess-kinder”, or loess dolls) in the upper part of loess deposits along the Missouri and Mississippi Rivers can be found remaining on rain-washed slopes. Limy, mudstone concretions and brown iron carbonate concretions are abundant in certain localities in northwestern and southeastern Missouri, where they are used as oddities in rock gardens or walls. [Illustration: Concretions. Dark, limy concretion at left and brassy pyrite at center and right. Note the inter-grown pair in center.] Some concretions are formed at the same general time as the surrounding rock accumulates, but others may be formed years after the surrounding rock has been buried or removed from the environment of its formation. In either case, deposition of the mineral matter follows the pattern of addition or “growth” from inside out. This growth, of course, does not involve a life process like that of a plant or animal. If two or more centers of deposition occur close together, the several growing concretions may touch, intergrow, and develop some weird forms, suggesting organic growth. However odd these curiosities may become, there is no question that they are not fossils, or evidence of life. Probably concretions excite the interest of persons more than any other rock structure.