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From an examination of such outcrops with special focus on the sequence of animal forms comes the empirical generalization that the faunas of the past have followed a specific order of succession, and so the relative age of a fossiliferous rock is indicated by the types of fossils it contains. Using fossils simply for identification purposes, Smith constructed a map of the various surface rocks outcropping throughout England, Wales, and southern Scotland.
Smith’s geologic map was extremely crude, but in its effect on Earth study it was a milestone.
The technique, called , involves lowering a small instrument down a drill hole on the end of a wire and making measurements continuously as the wire is played out in measured lengths.Presumably if all the world’s outcrops were integrated, sediments representing all of geologic time would be available for examination.This optimistic hope, however, must be tempered by the realization that much of the record—older than 541 million years—is missing.In addition, its fossilized population should be sufficiently abundant for discovery to be highly probable.Such an array of attributes represents an ideal, and much stratigraphic geology is rendered difficult because of departure of the natural fossil assemblage from this ideal.