The Relative Age of Concretions

The age of a concretion, relative to that of the parent-rock, must be either:— 1. Contemporaneous. —The concretion grew on the sea-floor at the same time, and at about the same rate, as the surrounding deposit was laid down. 2. Penecontemporaneous. —The concretion segregated more or less close to the surface of recently deposited sediment. 3. Subsequent. —The concretion formed after deposition had finished, and after the strata had largely consolidated, but, perhaps, before cementation had taken place to any noteworthy extent.