Biography from Strangescience.net
The earth has three different kinds of rocks. Igneous rocks are formed from melting rock, such as magma or lava. Sedimentary rocks — home to fossils — are broken down by wind or water into sediments that later solidify. Metamorphic rocks may have started as igneous or sedimentary rocks, but extreme heat or pressure has changed them into something different. In the rocks of Scotland, Hutton found fingers of granite reaching well into sedimentary rocks, and saw this as evidence of subterranean fire and heat. He surmised that the core of the planet could make new rock, offsetting the action of erosion.
He also found neatly deposited layers of sedimentary rocks overlaying rock layers that were almost vertical, as shown at right. The lower layers of rock, he concluded, must have been deposited eons before, then later upturned. In these unconformities between rock layers, Hutton saw evidence of vast expanses of time in earth's history. (Charles Lyell was following in Hutton's footsteps when he wrote his own masterwork in 1830.)
He also proposed a virtually limitless lifespan for our planet, and even though the currently accepted figure of roughly 4.5 billion years is beyond human comprehension, it's not infinite. In the 18th century, neither James Hutton nor anyone else could prove vast expanses of geologic time. Proof of the age of the earth would not come until the 20th century, when chemists learned how to estimate the ages of rocks through rates of radioactive decay.