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| Figure 5 Detail of a geological map of Philadelphia illustrated on page 10 of Dr. Gerhard Troost’s Geological Survey of the Environs of Philadelphia performed by the order of the Philadelphia Society for promoting Agriculture (Philadelphia: H. S. Tanner, 1826). (Courtesy, The Academy of Natural Sciences, Ewell Sale Stewart Library and the Albert M. Greenfield Digital Imaging Center for Collections.) |
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