The concretions under consideration are unusual in that they contain an exceptionally high percentage of uncombined alumina in an anhydrous state or in a very low state of hydration. Their present chemical character has been developed largely through the influence of compounds liberated in the decomposition of crabs about which they were formed. Their physical characters have been determined by internal forces due to chemical reactions, external pressures due to crustal movements, and variation in the permeability of the inclosing clays in different directions. The chemical and physical forces traced in this study are believed to be typical of many of those involved in the development of concretions of more common chemical and mineralogical character.

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