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By descending into mines, Smith was able to study the rock layers, or strata, noting their composition and the kinds of fossils preserved in them.This led him to conclude that the order of strata and fossils was consistent even in different places.In the mid-seventeenth century, James Ussher (1581–1656), the Archbishop of Ireland, compiled a chronology of Earth by adding up the generations named in the Bible.

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His chronology represented the common belief among Christians of his time that biblical events, including the creation account in Genesis, happened exactly as they were written.

By the nineteenth century it had become a popular opinion among scientists and scholars that Earth was created in a single event and that its short history was altered only by the great biblical flood.

Scientists know today that only igneous rocks are formed from a molten state, and only sedimentary rocks are deposited under bodies of water.

One enduring idea that James Hutton introduced, however, was the principle of uniformitarianism.

He divided the many different kinds of rocks that he studied into four broad categories: Rocks from the primary age, consisting of igneous or metamorphic rocks at the cores of mountains, were the first to be deposited.

Rocks from the secondary age were sedimentary layers deposited on the sides of mountains, on top of primary layers.

Deposits from the tertiary age were made up of hills of gravel and sand, i.e., broken pieces of rocks that were formed in earlier ages.

The quaternary age is the current era, in which rocks and soils are still being deposited.

Ussher's chronology was widely accepted at the time that early geologic investigators began their work.

In the 1750s, however, Giovanni Arduino (1714–1795), an Italian professor studying mining and surveying, began to realize that different kinds of rocks had been deposited at different times in history.

This would make the world about 5,650 years old in Ussher's day and about 6,000 years old now.

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