French and North American cherts used for millstones during the late eighteenth century and nineteenth century in Ohio are generally similar in color, hardness, presence of cellular porosity, and other characters. Although basic differences in fossil content and geologic age have been known since the 1830s, and previous authors have noted the presence of fusulinids and other fossils in Ohio buhrstone, and charophytes as well as gastropods in millstone pieces found in New England but imported from France, little attention has been paid to the fossil content of chert millstones in the United States. We document examples of chert millstones in Ohio, distinguishing cherts (French buhr) imported from the Tertiary of France for use as millstones from local Pennsylvanian cherts (buhrstone) used for millstones. French buhr (known as meulière in France), from the Tertiary deposits of the Paris Basin, contains gyrogonites belonging to the Oligocene species Gyrogona medicaginula, charophyte thalli, and freshwater gastropods. The fossil content of the Ohio cherts, as observed in millstones, is diverse, including Pennsylvanian (late Carboniferous) marine fossils, notably fusulinids, pelmatozoans. and brachiopods. The concepts and examples described in this paper are broadly applicable outside of Ohio as millstones made of Ohio chert were exported outside of Ohio to other states, and French millstones have been distributed worldwide, so these concepts can be applied to study of millstones (as well as other chert objects), found outside of North America as well. This study also serves as another example of the usefulness of steinkerns and molds of gyrogonites in identification of charophyte species.

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