The exposed stratified rocks of central Iowa are made up chiefly of Lower Coal Measure clays, shales, and sandstones. In the southeastern portion of the area the upper member (for Iowa) of the Sub-Carboniferous—the St. Louis limestone—is exposed along the Des Moines river. To the westward the so-called Middle Coal Measures and the Upper Coal Measures are represented. Hitherto it has been supposed that the three recognized divisions of the upper Carboniferous rocks in the state have each a maximum thickness of about two hundred feet. Lately, however, the Upper Coal Measures alone have been discovered to have at least double this estimate; and at a still later date the vertical extent of the other two formations has been found to differ very much from the limit usually assigned: the Middle Coal Measures being considerably thinner than was supposed, and the Lower Coal Measures very much thicker. . . .