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(or megachron), in geology, the largest stages in the tectonic history of the earth, lasting many hundreds of millions of years. In 1944 the German geologist H. Stille divided the history of the earth into three megacycles: Protogei, Deuterogei, and Neogei. The first of these corresponds to the early Precambrian (Archean), the second to the Middle Precambrian, and the third to the late Precambrian and the Phanerozoic. The term “Neogei” has become best known.