Concretions, Or Nodules, are developed after the formation of strata. They are balls or irregular lumps of a material differing from that of the stratum in which they occur. They are not pebbles, which are older than the stratum which contains them and which were embedded just as we find them, but are younger than the stratum and were formed subsequently. This is shown by the fact that the planes of stratification may often be traced through the concretion, and that fossils are sometimes found partly within and partly without the nodule. In shape the concretions vary greatly, from almost true spheres, to grotesque aggregations, but always with rounded form, and almost as great a variety of material is found among them. Very often a foreign body, like a fossil shell or leaf, forms the centre or nucleus of the nodule, which has been built up, often in concentric layers, around the nucleus. One form of concretion, known as a sepiarium, is divided internally by radial cracks, which were subsequently filled up with some mineral deposited from solution by percolating waters.

Concretions in Laramie sandstone, exposed by weathering.

Fig. 151. - Concretions in Laramie sandstone, exposed by weathering.

(U. S. G. S).

The agency which produces concretions cannot as yet be explained. The material of which they are made must have been scattered through the stratum and then gathered together at a later period. Such nodules have been observed in the process of formation in modern sediments, and it has further been noticed that when finely powdered substances are mixed together, certain of them do segregate into lumps. These observations, however, merely confirm the conclusion that concretions are due to segregation of scattered material in the stratum; they give us no explanation of the fact.

Ironstone concretion, split open to show the fossil leaf which forms the nucleus; Mazon Creek, Illinois.

Fig. 152. - Ironstone concretion, split open to show the fossil leaf which forms the nucleus; Mazon Creek, Illinois.

The commonest concretions are those of clay in various kinds of rock, of flint and chert in limestone, and of ironstone in clay rocks.