Twenty interbedded fluvial muds and sands (mud-sand pairs) from various rivers in Oklahoma, Missouri, Arkansas, Louisiana, Mississippi and Texas were collected and analyzed. Chemical treatments were used to isolate the solid silica (quartz plus chert) and feldspar from the sediments and point counts of grain mounts were made to determine the amounts of quartz, chert and types of feldspars in each sample. Atomic absorption spectrophotometry was used to obtain normative percentages of orthoclase, albite and anorthite. A combination of sieving and settling tube techniques was used to obtain the size distribution of solid silica plus feldspar fraction of each sample. In the samples analyzed, feldspar is finer grained than quartz plus chert and, therefore, the F/Q + C + F ratio in a mud is higher than the ratio in the associated sand. Consideration of a single size fraction (4.0-4.5 phi ) common to both the mud and sand members of the mud-sand pairs eliminated the influence of size on the ratio. Within the 4.0-4.5phi size fraction the F/Q + C + F ratios for the mud and sand of a mud-sand pair are the same. The chert grains in our chert-rich samples are coarser-grained than the associated quartz grains so that the chert/solid silica ratios are larger in the sands than in the muds. However, as with the feldspar grains the ratios are the same if only the 4.0-4.5phi size fraction of each sediment is considered. Most sandstones and almost all mudrocks contain sediment in the 4.0-4.5phi size fraction and this study has established that the F/F + Q + C and C/Q + C ratios in this size fraction are the same in these two types of sediments at the time of deposition. This establishes a baseline against which the feldspar, quartz, and chert contents of ancient rocks can be compared to determine, for example, whether feldspar has been preferentially removed from sandstones during diagenesis; or whether authigenic chert has been formed in mudrocks.