Surveying the Nomenclature of Geologic Time

Publication in 1976 of the International Stratigraphic Guide climaxed a movement toward unification of nomenclature begun a century earlier when an American committee formed to promote an international meeting of geologists. American stratigraphic practice during the remainder of the 19th Century developed chiefly through the needs of the United States Geological Survey for uniform standards in geologic nomenclature and cartography. The requirement for maps which were usable by the intelligent layman for practical purposes led Directors King and Powell to emphasize the mapping of lithology, rather than time units delineated faunally. This approach was not universally accepted and led to bitter dispute. H. S. Williams and C. D. Walcott deserve credit for clarifying the distinction between terms for time and terms for rocks. Under Director Walcott, the 1890 codification of Powell was modified and formalized into the 1904 "Rules" of the U.S. Geological Survey. A major distinction was the placing in separate sections those rules concerned with lithologic units and those concerned with time and correlation. This dual classification, fundamental to all United States and North American stratigraphic codes during this century, has become a guiding principal in the international efforts at standardization of usage.