The Cincinnati arch is commonly considered among geologists to date from the Ordovician. This concept is reflected in the statements of some current historical geology textbooks. However, a historical review of the concept shows that the Ordovician date was based on very tenuous evidence and that most early workers, such as Newberry, Orton, and Foerste, believed it to date from not earlier than the Silurian. The problem of its existence in Late Ordovician time was investigated by a thorough mineralogical, chemical, and gravitational methods. One hundred and fifteen shale specimens were collected laterally from three horizons within the Cincinnatian Series in its exposure area in southwestern Ohio, southeastern Indiana, and northern Kentucky. Twenty-two mineralogical, chem cal, and textural parameters were measured from each specimen and the data were treated statistically by an analysis of variance method. The calculation was performed by computer. The results indicated only a weak possibility of a embryonic arch being present during Early Cincinnatian time (Fulton). There is definite evidence of an east-west barrier unrelated to the trend of the Cincinnati arch dominating the basin of deposition during at least a part of early Late (Clarksville) Cincinnatian time. There is some evidence that a basin limiting element generally parallel with the previously proposed Cincinnati arch was influencing shale deposition at the close of Cincinnatian time. Its influence, however, was not strong and it is doubtful that it represents the type of paleogeographic featu e which is known to have developed in later Paleozoic time as the Cincinnati arch.