1890 - 1965
Arthur Holmes began studying physics at the Imperial College of Science in London, but switched to geology before graduating in 1910. In 1913, before he even earned his doctoral degree, he proposed the first geological time scale, based on the fairly recently discovered phenomenon of radioactivity. Using his quantititive time scale and other factors, he made an estimate of Earth's age that was far older than anyone had suggested until then -- 4 billion years. His initial estimates of Earth's eras have held up remarkably well over time: For example, he placed the beginning of the Cambrian period at around 600 million years ago; today 590 million years is the time frame largely accepted.
Around 1930, Holmes suggested a mechanism that could explain Alfred Wegener's theory of continental drift: the power of convection. Currents of heat and thermal expansion in the Earth's mantle, he suggested, could force the continents toward or away from one another, creating new ocean floor and building mountain ranges (a theory later clarified by Harry Hess). Holmes was a widely respected geologist by then, but he was a few years too late to support Wegener (who died in 1930), and about 30 years too early to have hard data to back up his theory. He warned that his ideas were "purely speculative" and could "have no scientific value until they acquire support from independent evidence." Yet he had come very close to describing the modern view of Earth's plates and the dynamics between them.
Holmes was professor of geology at the universities of Durham and Edinburgh until his death in 1965.