In 1895, John Perry produced an age-of-Earth estimate of 2 to 3 billion years using a model of a convective mantle and thin crust.Kelvin stuck by his estimate of 100 million years, and later reduced it to about 20 million years.

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The discovery of radioactivity introduced another factor in the calculation.

After Henri Becquerel's initial discovery in 1896, Marie and Pierre Curie discovered the radioactive elements polonium and radium in 1898; and in 1903, Pierre Curie and Albert Laborde announced that radium produces enough heat to melt its own weight in ice in less than an hour.

These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.

In the mid-18th century, the naturalist Mikhail Lomonosov suggested that Earth had been created separately from, and several hundred thousand years before, the rest of the universe. In 1779 the Comte du Buffon tried to obtain a value for the age of Earth using an experiment: He created a small globe that resembled Earth in composition and then measured its rate of cooling.

Their values were consistent with Thomson's calculations.

However, they assumed that the Sun was only glowing from the heat of its gravitational contraction.

Comparing the mass and luminosity of the Sun to those of other stars, it appears that the Solar System cannot be much older than those rocks.

Calcium-aluminium-rich inclusions—the oldest known solid constituents within meteorites that are formed within the Solar System—are 4.567 billion years old, giving an age for the Solar System and an upper limit for the age of Earth.

This led him to estimate that Earth was about 75,000 years old.

Other naturalists used these hypotheses to construct a history of Earth, though their timelines were inexact as they did not know how long it took to lay down stratigraphic layers.

In 1830, geologist Charles Lyell, developing ideas found in James Hutton's works, popularized the concept that the features of Earth were in perpetual change, eroding and reforming continuously, and the rate of this change was roughly constant.