New information on the nature of the sandstones of the lower part of the McLeansboro group of the Pennsylvanian system in a part of southern Illinois has resulted from recent diamond drilling in northern Williamson County, in the southern Illinois coal field (fig. I). A sandstone which crops out in the immediate vicinity of the bore holes is the uppermost sandstone encountered in the cores, and a definite correlation can be made from surface exposures to the subsurface section. Two sets of samples of this sandstone were collected, one from the drill holes, the other from the outcrop. Thin sections were cut and heavy mineral analyses made from these samples. The sandstone sampled is the basal member of the cyclothem containing No. 8 coal. It lies unconformably on the uppermost shale member of the cyclothem next below. Its base is about 200 feet above the base of the McLeansboro group. Kosanke and Eddings (1946, p. 1212) have correlated coal No. 8 of southern Illinois with the Trivoli (No. 8) coal of western Illinois. Thus the basal sandstone member of the Williamson County cyclothem presumably can be correlated with the basal member of the Trivoli cyclothem, the Trivoli sandstone. Cooper (1945, p. I6) has correlated the Trivoli sandstone of western Illinois with the basal member of the Missouri series of the MidContinent section, lying just above a major floral and faunal break in the Pennsylvanian