Concretions of various kinds are abundant in several formations in the Great Plains region, and their prominence is so marked that facts which throw light on their origin are greatly to be desired. Recently there has been discovered in one of the Tertiary formations an exposure which exhibits in a most instructive manner the development of concretions by evident crystallization, which it is the purpose of this paper to describe.
The Tertiary of the Great Plains region comprises the White River group of the Bad lands, mainly of Oligocene age, and a great series of deposits which originally were included under the term Loup Fork. The greater portion of this series, extending through Nebraska into South Dakota, has been separated by Darton under the designation of Arikaree formation. It is believed to be of Miocene age. This formation consists mainly of sand which locally is consolidated . . .