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Following Smith’s pioneering work, generations of geologists have confirmed that similar and even more extensive fossil sequences exist elsewhere.
To this day, fossils are useful as correlation tools to geologists specializing in stratigraphy.
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.
That is to say, it depends on other sequence-determining principles for establishing its validity.Failing that, the repetition of a certain layered sequence (e.g., a black shale sandwiched between a red sandstone and a white limestone) lends confidence to physical correlation.Finally, the measurement of a host of rock properties may well be the ultimate key to correlation of separated outcrops.In other words, the fossil species must have had a short temporal range.On the practical side, an index fossil should be distinctive in appearance so as to prevent misidentification, and it should be cosmopolitan both as to geography and as to rock type.