Quartz sand of fine to medium size, well sorted, rounded, and frosted is common and widespreadin many units of Early and Middle Paleozoic age in the central part of North America. A detailed study of its distribution in Silurian and Devonian rocks in the region extending from the Cincinnati arch across the Illinois basin to the Wisconsin and Ozark Highlands shows that, except as cavity filings, sand is not present in the Middle and Upper Silurian (Niagaran and Cayugan) or in Devonian rocks equivalent to or older than the Camden and Clear Creek Cherts. Characteristically, sand occurs in varying concentrations in Middle Devonian rocks immediately overlying the erosion surface that truncates Silurian and Lower Devonian (Clear Creek and earlier) strata. It also occurs in shoreline concentrations marking subsequent retreats and advances of the Devonian sea and in the extent of each onlap over the older rocks. The vertical repetition of similar-appearing sandstones and sandy zones has led to misidentification and iniscorrelation of stratigraphic units at many places in the area studied. The patterns of distribution identify the sources of the sand and aid in interpreting the behavior of a part of the North American craton during much of Devonian time.