Pottsville and Allegheny underclays show significant between-bed variation. The high alumina clays of the Pottsville and lower Allegheny are the lateral facies equivalents of marine limestones or shales which contain relatively abundant illite, chlorite, Fe 2 O 3 , and silica. Paleogeographically, the high alumina clays are located in seampy areas on abandoned clastic wedges fringed by shallow seas. It is concluded that both stratigraphic and geographic variations in the clay mineral composition of underclays is largely controlled by the chemistry of the depositional environment.

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