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Figure 1.8 Proof of extinction: Cuvier's comparison of (a) the lower jaw of a mammoth and (b) a modern Indian elephant. (Courtesy of Eric Buffetaut.)

documented the history of long spans of time. Until the late 18th century, scientists accepted calculations from the Bible that the Earth was only 6000-8000 years old. This view was challenged, and most thinkers accepted an unknown, but vast, age for the Earth by the 1830s (see p. 23).

The geological periods and eras were named through the 1820s and 1830s, and geologists realized they could use fossils to recognize all major sedimentary rock units, and that these rock units ran in a predictable sequence everywhere in the world. These were the key steps in the foundations of stratigraphy, an understanding of geologic time (see p. 24).

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