An unconformity at the base of the Coal Measures in the upper Mississippi valley has long been known. In the writings of the early geological explorers are recorded observations showing that a physical break in the stratigraphic sequence was recognized at various localities between the coal-bearing strata and the underlying Carboniferous limestones. In most cases it was regarded as local discordance in sedimentation. Hall* appears to be the first to note the wide extent of the unconformity. Later, White f also emphasized its importance as a stratigraphic plane. Of recent years much information has been obtained regarding the nature of this junction of the coal-bearing Middle Carboniferous strata and the formations on which they rest. The details of the unconformity plane have been so nicely worked out in so many localities that we have now a very good idea of its character, its real significance, and even of . . .