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Scientist realized that fossils occurred in consistent order. The study of the succession of fossils and its radioactive dating is called biostratigraphy. Each increment of time can be characterized by the assemblage of different fossil organisms. These zones can be traced regionally and then globally.

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http://www.crowcanyon.org/images/graphics/stratigraphic_profile.jpg
http://www.globalchange.umich.edu/fossil1.jpg

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4 A structure that cuts another is younger than the structure that it cut.
5 A structure that is included in another is older than the including structure.
6. Process operating in the past were defined by the same laws of physics.

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There are six principles for stratigraphic superstition. 1. A higher rock unit is younger than a lower one.
2. Rock layers were originally deposited close to the horizontal.
3. A rock layer continues laterally unless their is a structure or change that prevents its extension.

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http://dnr.louisiana.gov/assets/TAD/education/BGBB/1/measuring.html
http://image.slidesharecdn.com/originsoflife3-ageofearth-130223212739-phpapp02/95/origins-of-life-3-age-of-earth-3-638.jpg?cb=1361654930

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The discovery of natural radioactive decay in 1896 opened the door. In 1905, Lord Rutherford made the first prediction of how old the Earth was by using radioactivity. This allow scientist to prove how old fossils and rocks are.

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Scientists use three techniques to figure out the Earth's age. Scientist use radiation measurement, stratigraphic superstition, and the fossil record. The scientists have discovered that the Earth is about 4.6 billion years old.