The abundance of feldspar as related to grain size has been examined in quartz-rich cratonic sandstones of Cambrian, Ordovician, Pennsylvanian-Permian and Jurassic ages, which have traditionally been characterized as supermature quartz arenites. Feldspar contents ranged from less than 1% to more than 50%. In the Cambrian sandstones of the upper Mississippi Valley, detrital feldspar grains are almost invariably < .125 mm. Due to the great abundance of feldspar, fine and very fine-grained sandstones in this sequence are either feldspathic (10-25% feldspar) or highly feldspathic (25-65% feldspar). The percentage of feldspar is nearly linearly related to the volume of the very fine sand fraction. The St. Peter Sandstone (Ordovician) usually contains less than 2% detrital feldspar, but larger amounts occur where it contains significant amounts of very fine sand and coarse silt. Feldspar in the St. Peter is concentrated in the coarse silt size fraction rather than in the very fine sand fraction as in Cambrian sandstones. This may indicate intensive abrasion of initially small feldspar derived from Cambrian sediments. The Weber Sandstone (Pennsylvanian-Permian) contains up to 20% feldspar whose abundance also increases with decreasing mean size, being most abundant in samples with means below < .125 mm. In samples of the Navajo Sandstone (Jurassic), feldspar is relatively evenly distributed among the size fractions < .35 mm, yet the highest feldspar concentration occurs in the .125 to .044 mm size range. The distribution of feldspar within various size fractions of sandstones is significant in making provenance studies, in explaining regional and stratigraphic mineralogical variations, in interpreting depositional environments, and in assessing the origin of compositional maturity or immaturity.