Although radiometric dating may at some time in the future provide data from which to construct a calendar of geologic time, the chronologic foundation of the present geologic time scale has been, and is, fossils. In reality the unit of stratigraphic classification is the "time unit" (series and stage) and not the "time-stratigraphic unit." From the "time-stratigraphic unit" we have derived the concept of the "time unit." The idea of the type section has been strongly emphasized in the United States from an early date. The type section for a rock-stratigraphic unit is at best only "typical" of what the original describer saw and should not be used to freeze the concept intended. Paradoxically, although we give lip-service to an awareness of isochronous but different facies when we write, in practice we say that similarity implies isochroneity - in other words, the doctrine of uniformitarianism becomes the doctrine of uniformity. A long distance today between homotaxial sequences can support the hypothesis of isochroneity or the hypothesis of continental drift without proving either one.

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