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The father of modern geology did not start out as a geologist. He first apprenticed with a lawyer and then earned a degree in medicine. But after he inherited some land, he decided to devote himself to agriculture, and this led him to think about the origin of soil and its relation to the rest of the landscape, especially the rocks. Based upon his observations, he concluded that old rocks are pushed upwards to become mountains, that new rocks form from the emergence and solidification of lava, and that the driving energy for both of these processes must be the Earth's internal heat. He also concluded that soil forms from rocks through the long process of weathering. In this way Hutton developed the idea that the soil, the rocks, and the landscape were all connected in a single process, which he called Plutonism, in honor of Pluto, the Greek god of the underworld. Hutton realized that the cycle of uplift and erosion required a long time and that the Earth must therefore be much older than a few thousand years, as was widely believed at the time. But it was not until the twentieth century that Hutton's theory was proven correct when geologists, using a technique called radiometric dating, demonstrated that the Earth is in fact more than four billion years old.