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These three woods undoubtedly ranged over Western America in Tertiary and were well represented in the Ginkgo series of forest during Miocene. Fossil walnut wood is the more abundant of the three species, while hickory is not common.
Beck, G. F (1937). Determination of Fossil Woods (Part IV-Walnut-Hickory-Persimon). The Mineralogist, 5(4), 7-8.
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