Cyclic sedimentary sequences of similar origin but diverse manifestations are widespread in later Mississippian, Pennsylvanian, and early Permian successions of the northern hemisphere. They are best developed in cratonic areas, and vary from wholly marine through marine-non-marine alternations, to wholly non-marine. They are associated with widespread coals in many areas, but are equally evident in successions lacking coal.

Their origin has been assigned to (1) intermittent downwarping of sedimentary basins; (2) continuous downwarping; (3) periodic elevation and depression of both source areas and basins; (4) eustatic changes of sea-level associated with (a) alternate growth and wastage of late Paleozoic glaciers in the southern hemisphere and in India, or (b) controlled by diastrophism in the ocean basins; (5) climatic oscillations, and (6) the superposition of subdeltas such as those of the Quaternary in Louisiana.

The present paper attempts to resolve their origin by (1) study of the varied tectonic conditions in areas displaying cyclic successions; (2) environmental mapping of the separate beds of cyclothems through several states; (3) determination of the time and frequency of late Paleozoic glaciers; and (4) critical appraisal of the various mechanisms proposed.

The kinds and quantities of sediment composing a cyclothem at any place are held to be controlled by (1) rate of downwarping; (2) type and quantity of clastic sediment available; (3) distance between sediment source and basin. The varied number of cyclothems in basins and adjacent shelves suggests that many nondepositional periods occurred both between and within cycles.

Eustatic shifts in sea-level through 100 feet or less, controlled by glacial episodes or submarine diastrophism, and climatic oscillations, are judged to be the most probable causes of these cycles.

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