Fig. 370.—Rain-drop impressions. (Brigham.)
Concretionary structure.—Various sedimentary formations contain nodules or irregularly shaped masses of mineral matter unlike the rock in which they occur. When these nodules consist of matter aggregated about some center, they are called concretions. They are common in sedimentary rocks, and here it may sometimes be seen that the aggregation has taken place about a shell, a leaf, or some other organic relic. The nuclei are, however, not always organic. The material of the concretion may have come from the immediately surrounding rock, having been first dissolved by water and then deposited about the nucleus, or it may have been introduced from without, likewise by the agency of water. In the first case, the mineral matter of the concretion is usually one of the minor constituents of the rock. Thus the commonest concretions in limestone are composed of impure silica (chert, [Fig. 361]); in shale, of lime carbonate or iron sulphide; in sandstone, of iron oxide. The concretion may be made up almost wholly of concentrated matter, in which case the matter originally in the place of the concretion has been crowded aside; or it may involve much of the material of the imbedding rock. Thus the concretion of lime carbonate in shale may be nearly pure, or it may involve much of the earthy matter of the shale, while the concretion of iron oxide in sandstone commonly includes much sand. In extreme cases, indeed, the concentrated matter of the concretion merely cements the material involved into distinct nodules. Occasionally the rock substance itself takes on a concretionary form, all or most of its material being involved.
Fig. 371.—Discoid calcareous concretions from post-glacial clays. Ryegate, Vt. (Photo. by Church.)
Fig. 372.—Irregular calcareous concretions. Ryegate, Vt. (Photo. by Church.)
Fig. 373.—Calcareous concretions, some of them showing bilateral symmetry. Ryegate, Vt. (Photo. by Church.)
Fig. 374.—Irregular tubular silicious concretions in Arikaree clays. Northwest of Wildcat Mountain, Banner Co., Neb. (Darton, U. S. Geol. Surv.)