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In the 1700's belief in a 6000 year old Earth crumbled.
Attempts to calculate the age of the Earth from physical considerations yielded estimates that ranged from 75,000 years (Buffon, 1774) to several billion years (de Maillet, Buffon).
Notable observations included: ran from about 1780-1850.
By the end of the 18'th century it was clear that the Earth had a long and varied history. The major debate was between the catastrophists, e.g., Cuvier, who held that the history of Earth was dominated by major catastrophic revolutions and the uniformitarians, e.g.
The rise of science produced a major change in attitude.
There was no single estimate of the Earth's age in the mid 1800's and no good way to arrive at one.
The account in Genesis is replete with miracles that do not stand up under rational analysis.
This did not matter; the theological perspective did not require physical rationalization.
Ussher accepted the Biblical account at face value, relying on the Biblical genealogies and on extant historical records.
He implicitly assumed that the world was created much as it is now.
The great debate was won by the uniformitarians, so much so that the degree of gradualism was overstated and the importance of catastrophes was unduly minimized.