The first hint that the Earth is very old came from a volcano.
Charles Lyell visited Mount Etna in 1828.
Mount Etna is the largest active volcano in Europe.
It covers a huge area (38 x 47 km) and has a maximum height of nearly 3.5 km.
The volcano dominates the landscape of North-eastern Sicily, Italy.
Lyell saw that it was made of many lava flows, piled one on another.
This suggested it had existed for a very long time.
But when he found a limestone layer that ran beneath the oldest lava, he found that 95% of the fossils represented living species.
He knew from the work of William Smith that the percentage of extinct fossil species in a rock was an indicator of the rock’s age.
For example, if 50% of the fossils were of extinct species, it was of moderate age.
Fossil scallops in a limestone layer near Etna, alongside modern scallops.
With so many modern types of fossil in the rocks under Etna, in geological terms Etna was very young.
Yet in human terms, it appeared ancient.
Lyell had discovered that human time and geological time were not comparable.
He now realised the Earth must be immensely old in human terms.
Lyell's "Principles of Geology" is a classic of 19th century science.