In contrast to the previously interpreted two-phase cycle (transgression-regression), an appreciable number of typical Pennsylvanian cyclothems in widely separated areas manifest the four following rapid, eustatic, base-level reversals superimposed on a simple tectonic environment of continuous slow subsidence.

1. Base-level strongly down; maximum marine withdrawal and regional sub-aerial erosion (channeling); local peat near receded shoreline.

2. Base-level moderately up; moderate marine advance; resumption of marine sedimentation basinward (e.g., Western Interior) and initiation of fluvial deposition landward (e.g., Eastern Interior).

3. Base-level moderately down; marine withdrawal again to lowest position, but relief insufficient for significant erosion; low, vegetated surface decomposed (underclay) as it slowly subsides into swamp (widespread peat).

4. Base-level strongly up; maximum marine advance; marine deposition resumed basinward (Western Interior) and initiated landward (Eastern Interior) in upper hemicyclothem; landward margin of basin (e.g., New Brunswick) characterized by red fluvial deposits, indicating aridity.

These four cyclic phases coincide with, and appear to be correlatives of, the four following phases of Simpson's solar radiation glacial cycle, respectively.

a. First glacial phase; maximum water in ice, vegetation, subsurface, atmosphere, lakes, and streams; minimum in seas (lowest sea-level).

b. First interglacial phase (short, warm, and wet); much water in vegetation, subsurface, atmosphere, lakes, and streams, but little in ice (intermediate sea-level).

c. Second glacial phase; maximum water in ice, vegetation, subsurface, atmosphere, lakes, and streams; minimum in seas (lowest sea-level).

(d) Second interglacial phase (long, cool, and markedly arid); minimum water in ice, vegetation, subsurface, atmosphere, lakes, and streams; maximum in seas (highest sea-level).

Because Simpson's theory or its modifications demand eustatic fluctuations and climatic changes of the same magnitude and sequential order as those indicated by late Paleozoic stratigraphy, the concept of eustatics as a primary factor in cyclothemic control and Simpson's glacial theory are mutually supported.