Chemical composition and mineralogy of a large number of samples of molluscan shell material from limy concretions in the Fox Hills Formation (Cretaceous) of South Dakota were determined. The results show an extremely wide range of concentrations for the minor elements Mg, Fe, Mn, Sr, and Ba and of calcite-aragonite ratios.

Detailed studies of an ammonite, Sphenodiscus, a characteristic faunal element of this formation, showed that the range of minor-element concentrations could best be interpreted as the consequence of at least two separate alteration processes, involving contamination and recrystallization, probably during diagenesis. Neither visual nor mineralogic criteria can be used to predict the degree of alteration of the original shell composition. Since the alteration processes may have both added and subtracted material, one must be cautious in accepting any fossil-shell-composition data as representative of the originally deposited test.

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