Concretions do not lend themselves to the exact methods of description used by the mineralogist. Paleontologists are likely to mention them only casually in connection with the fossils which they often inclose. Other geologists are apt to let them severely alone save for the briefest possible mention. Concretions may therefore be said to belong to a sort of “no-man’s-land” in geology. Notwithstanding excellent descriptions of particular kinds of concretions which have appeared, we are still without criteria which will enable geologists to agree as to whether certain forms are of organic or inorganic origin. Cases have been recorded where structures of undoubted concretionary origin have been considered fossils by some geologists.2 A number of forms which have been described as fossils are considered by some competent students of concretions to be partially or entirely due to inorganic agencies. As long as diversity of opinion exists as to whether a . . .

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