The results of a detailed petrologic investigation of underclays of various types occurring beneath the coals in Illinois are presented. The mineral composition of the clay grade was determined by optical, X-ray, and chemical criteria, frequently following sedimentation and supercentrifuge separations into fractions sufficiently simple mineralogically so that the analytical data could be definitely interpreted. Constituents coarser than clay were identified with the petrographic microscope. Base-exchange capacity values and textural characteristics are presented and discussed.

The completely noncalcareous underclays occurring most commonly beneath coal No. 2 and older coals are composed largely of kaolinite. Varying amounts of illite and quartz are also present. Occasional beds are found in these underclays which are composed almost wholly of a distinctive member of the illite clay mineral group. The calcareous underclays and those grading from noncalcareous to calcareous which occur beneath younger coals contain illite as the essential clay mineral constituent. Except for variations in the calcite and quartz content and a slight concentration of limonite in a zone several inches below the coal, there is no evidence of variation in the mineral constitution or chemical composition within beds of the latter underclays as they are traced downward from the overlying coal. The analytical data definitely indicate no appreciable vertical variation of the completely noncalcareous underclays.

Underclays seemingly have been subjected little or not at all to processes similar to those forming modem soils. The significance of the petrographic data on the origin and genesis of underclays is discussed in detail.

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