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The Eras of Geologic Time

             5 billion years or more -- according to recent estimates. This vast span of time, called geologic time by earth scientists, is difficult to comprehend in the familiar time units of months and years, or even centuries. How then do scientists reckon geologic time, and why do they believe the Earth is so old? A great part of the secret of the Earth's age is locked up in its rocks, and our centuries-old search for the key led to the beginning and nourished the growth of geologic science.
             Geologic time is often discussed in two forms: .
             • Relative time ("chronostratic") -- subdivisions of the Earth's geology in a specific order based upon relative age relationships (most commonly, vertical/stratigraphic position). These subdivisions are given names, most of which can be recognized globally, usually on the basis of fossils. .
             • Absolute time ("chronometric") -- numerical ages in "millions of years" or some other measurement. These are most commonly obtained by using radiometric dating methods performed on appropriate rock types. .
             Some of the most important discoveries of modern science and technology have been the discovery of the Earth's age and the vast length of time encompassed by its history. This scale, comprised of billions and trillions of years is called the geologic time. In the mid-17th century an Irish clergyman, Bishop James Usher, added the years in the biblical genealogies and concluded that the Earth was created in 4004 BC. This idea persisted for a long while, although the 18th-century French scientist Georges Louis Leclerc, Comte de Buffon, reasoned that the Earth cooled from an originally molten body and that this would have required at least 75,000 years. .
             Buffon had to recant, but development of the principle of uniformitarianism in the late 1700s and early 1800s provided geologists with new grounds for arguing that the Earth is far older than anyone had imagined.

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