Geologic age dating sequence

Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.

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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.

The basis for assuming that like fossils indicate contemporary formation is faunal succession.

However, as previously noted, times of volcanism and metamorphism, which are both critical parts of global processes, cannot be correlated by fossil content.

Correlation based on the physical features of the rock record also has been used with some success, but it is restricted to small areas that generally extend no more than several hundred kilometres.

The first step is determining whether similar beds in separated outcrops can actually be traced laterally until they are seen to be part of the same original layer.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.Unlike the principles of superposition and crosscutting, faunal succession is a secondary principle.That is to say, it depends on other sequence-determining principles for establishing its validity.In other words, the fossil species must have had a short temporal range.

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