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Our transcription: Rock formations can be valuable time markers, but correlation is difficult over great distances because physical continuity and rock characteristics can be ambiguous. Fossils like these shells of marine organisms can be extremely valuable time markers. Each fossil species has its own life span, and rocks that contain fossils with overlapping life spans are actual time markers that can be recognized wherever they are found. The use of fossils in making geologic time correlations was first discovered in the Fifteenth Century. And by the beginning of this century, geologists had used correlation to extend the historic rock sequence from Europe to every continent on Earth.
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