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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.
Unlike the principles of superposition and crosscutting, faunal succession is a secondary principle.
Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.
Precambrian rocks must therefore be correlated by means of precise isotopic dating.
Suppose there exist a number of fossil-bearing outcrops each composed of sedimentary layers that can be arranged in relative order, primarily based on superposition.
Suppose, too, that all the layers contain a good representation of the animal life existing at the time of deposition.
In dating the past, the primary value of fossils lies within the principle of faunal succession: each interval of geologic history had a unique fauna that associates a given fossiliferous rock with that particular interval.
The basic conceptual tool for correlation by fossils is the index, or guide, fossil.
Ideally, an index fossil should be such as to guarantee that its presence in two separated rocks indicates their synchroneity.
This requires that the lifespan of the fossil species be but a moment of time relative to the immensity of geologic history.
With the development of miniaturized analytical equipment, evaluation of rock properties down a small drill hole has become possible.
The technique, called well logging, 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.
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.Tags: Adult Dating, affair dating, sex dating