Shales can be divided into several groups on the basis of their areal extent, uniformity or variability of lithologic properties, and lithologic associations. It is believed that the broader environmental features of shales can be reconstructed from areal patterns of their lithologic and faunal attributes, but the detailed environmental analysis of single samples must await further research on clay minerals and on conditions of deposition of modern muds. A sedimentary-tectonic grouping of shales is suggested as a basis for classification; and a rapid routine of shale examination is described which affords data for areal mapping.

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