"Theories of varying complexity which have sought to explain cyclic sedimentation are briefly summarized, and it is suggested that the two basic requirements are a slowly subsiding sedimentary basin and more or less regular eustatic changes of sea level. The former will decide the geographical location. The latter could be a natural, periodic and world-wide consequence of the combination of the continuous effects of sedimentation and isostatic or orogenic movements of the sea floor."

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