Concretions

Concretions, more or less isolated masses, spherical, elliptical, or irregularly nodular in form, occurring commonly in layers in stratified rocks, varying in size from minute particles up to masses several feet in diameter, and generally differing in composition from the rock in which they are imbedded. Thus, in alluvial clays and gravels we find fantastically irregular nodules of calcareous matter known as "race," "fairy-stones," or, on the Rhine, "loss-puppen"; in the Lias, Oxford, London and other clays we have the large "septaria" (q.v.) of impure limestone; in clays or in Chalk we have nodules of iron pyrites; and in the latter the well-known flints, resembling the siliceous phthanites and cherts occurring in other limestones. Concretions have been formed subsequently to the deposition of the rock in which they are, the lines of stratification often passing through them. They are often formed round a fossil as a nucleus, or have caught up one or more in the process of their formation, these fossils not being necessarily at all similar in composition to the nodule. That they have been produced by chemical aggregation is evidenced by the presence of less silica diffused as an impurity through chalk in the vicinity of lines of flint nodules than farther from them. The general law of their formation would seem to be that any chemical substance, existing in a small proportion diffused through the mass of a rock, is likely to be drawn together in lumps, especially round any solid body. Some rocks exhibit a concretionary structure throughout, as in the beds of clay-ironstone nodules in the Coal-measures, in oolite (q.v.), and in pisolite; and in other cases a flaking off of concentric shells is only manifested on the decay of the rock when exposed.