Abstract

The Cheltenham fire clay of Missouri is relatively massive in structure while typical shale, exemplified by the Lagonda, is laminated. The microscopical texture of the Cheltenham fire clay is a pattern of clay minerals grown in random directions, whereas that of the associated shale is an oriented arrangement of clay-mineral flakes which lie parallel to the lamination. It is deduced that the Cheltenham fire clay was developed from redeposited weathered-soil clay and further hydrolyzed and dialyzed in nearly stagnant water of Pennsylvanian swamps. The crystals of the fire-clay minerals grew at random in this clay gel. Many shales, on the other hand, are believed to have been deposited directly after flocculation without a significant interim period of leaching. The clay floccule, having a flaky shape, settled to the bottom where countless other clay flakes in the same orientation gave rise upon compaction to a conventional laminated shale. Possibly these criteria may be extended more generally to other argillaceous rocks.

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