Relative dating of sedimentary rocks
The universe is full of naturally occurring radioactive elements.
This idea is known as Plutonism, in reference to the God of the deep underworld.
By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as "bracketing" the age of the sedimentary layer in which the fossils occur.
Teach your students about absolute dating: Determining age of rocks and fossils, a classroom activity for grades 9-12.
But this explanation could not account for the fact that fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earth's surface.
Seeking an alternate explanation, other writers proposed that the fossils had formed within the rocks as a result of mysterious forces.