Johnson, Joseph Colin Francis (1897) Getting Gold: A Practical Treatise for Prospectors, Miners, and Students Lippincott, Philadelphia, Pennsylvania, page 15, OCLC5164023
Il tufo (PDF), su Archeologia - Metodologie. URL consultato il 16 febbraio 2019 (archiviato dall'url originale il 22 dicembre 2012).
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Johnson, Joseph Colin Francis (1897) Getting Gold: A Practical Treatise for Prospectors, Miners, and Students Lippincott, Philadelphia, Pennsylvania, page 15, OCLC5164023