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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.

References in periodicals archive ?
Open-water intervals at the base of megacycles 1 and 3 sharply overlie multistorey sandstones in a way consistent with the rapid flooding events interpreted by Browne and Plint (1994).
These relationships suggest that, at the type section, only some of the Boss Point megacycles record direct evidence of flooding and that these events could have been either rapid (megacycles 1 and 3) or gradual (megacycles 10, 13/14, and 15).
The upper limit of the Latest Jurassic to Early Cretaceous Megacycle 3 is an intra-Albian unconformity bounding the base of Late Cretaceous Megacycle 4 (Postrift stage 2; Salas et al.
It uses frequency modulated radio waves in definite frequency channels of 300 megacycles or higher.
The intercepted signals from the night-fighter told him it was transmitting on 492 megacycles and Bill Bigoray got the first message off to base.
System simulations run at up to 5 megacycles per second on popular desktop computers dependent on the number of components and their complexity.