He rejected the idea of a fixed age for the Earth, arguing that it was immensely old, but he didn't assign a numerical age. This was based on his observations and the principles of uniformitarianism - the idea that geological processes happening today have been happening at similar rates throughout history.

He rejected the prevailing view of his time, catastrophism, which argued for a young Earth based on the idea that major geological features were formed by sudden, catastrophic events.

His work was a major influence on Darwin's theory of evolution, providing a much longer timeframe for the gradual changes that Darwin observed.