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.
In the early 19th century, the scale of time in geological history was unknown.
Charles Lyell visited Etna in 1828.
He saw it was made of many lava flows, piled one on another.
This suggested it had existed for a long time.
But when he found a limestone layer that ran beneath the mountain, 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.
So if 50% of the fossils were extinct species, it was of moderate age.
Fossil scallops & modern scallops in a limestone layer near Etna.
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 knew the earth must be immensely old.
Lyell's "Principles of Geology" is a classic of 19th century science.