CONCRETION, in petrology, a name applied to nodular or irregularly shaped masses of various size occurring in a great variety of sedimentary rocks, differing in composition from the main mass of the rock, and in most cases obviously formed by some process which ensued after the rock was deposited. As these bodies present so many variations in composition and struc ture, it will conduce to clearness if some of the commonest be briefly described. In sandstones there are often hard rounded lumps, which separate out when the rock is broken or weathered. They are mostly siliceous, but sometimes calcareous, and may differ very little in general appearance from the bulk of the sand stone. Through them the bedding passes uninterrupted, thus show ing that they are not pebbles; often in their centres shells or fragments of plants are found. Argillaceous sandstones and flagstones very frequently contain "clay galls" or concretionary lumps richer in clay than the remainder of the rock. Nodules of pyrite and marcasite are common in many clays, sandstones and marls. Their outer surfaces are tuberculate; internally they com monly have a radiate fibrous structure. Usually they are covered with a dark bre :wn crust of limonite produced by weathering ; occasionally imperfect crystalline faces may bound them. Not infrequently (e.g., in the Gault) these pyritous nodules contain altered fossils.
Another type of concretion, very abundant in many clays and shales, is the "septarian nodule." These are usually flattened, disk-shaped or ovoid, often lobulate externally like the surface of a kidney. When split open they prove to be traversed by a network of cracks, which are usually filled with calcite and other minerals. These white infillings of the fissures resemble partitions; hence the name from the Latin septum, a partition. Sometimes the cracks are partly empty. They vary up to zin. in breadth, and are best seen when the nodule is cut through with a saw. These concretions may be calcareous or may consist of carbonate of iron. The former are. common in some beds of the London Clay, and were formerly used for making cement. The clay ironstone nodules or sphaerosiderites are very abundant in some Carboniferous shales, and have served as iron ores; some of the largest are 3ft. in diameter, and in the centre fossils are often found, e.g., coprolites, pieces of plants, fish teeth and scales. Phosphatic concretions are often present in certain limestones, clays, shelly sands, and marls.
Another very important series of concretionary structures are the flint nodules which occur in chalk, and the patches and bands of chert which are found in limestones. Flints consist of dark coloured cryptocrystalline silica. They weather grey or white by the removal of their more soluble portions by percolating water. Their shapes are exceedingly varied, and often they are studded with tubercules and nodosities. They sometimes have internal cavities and very frequently contain shells of echinoderms, mol luscs, etc., partly or entirely replaced by silica, but preserving their original forms. Chert occurs in bands and tabular masses rather than in nodules ; it often replaces considerable portions of a bed of limestone (as in the Carboniferous limestone). Corals and other fossils frequently occur in chert, and when sliced and microscopically examined both flint and chert often show silicified foraminifera, polyzoa, etc., and sponge spicules.
These examples will indicate the great variety of substances which may give rise to concretions, which seem to arise from the tendency of chemical compounds to be slowly dissolved by interstitial water, either while the deposit is unconsolidated or at a later period ; it is highly probable that such solutions are usually in the colloidal state. Certain nuclei, present in the rock, then determine reprecipitation of these solutions, and the deposit once begun goes on till either the supply of material for growth is exhausted, or the physical character of the bed is changed by pressure and consolidation till it is no longer favourable to further accretion. The process resembles the growth of a crystal in a solution by slowly attracting to itself molecules of suitable nature from the surrounding medium; but in the majority of cases it is not the crystalline forces, or not these alone, which attract the particles. The structure of a flint, for example, shows that the material had so little tendency to crystallize that it remained permanently in cryptocrystalline or subcrystalline state. That the concretions grew in the solid sediment is proved by the manner in which lines of bedding pass through them and not round them. (J. S. F.)