The distribution of extinction types in silstone from the Red Peak Formation (Early Triassic) of Wyoming and the Spearfish Formation (Permian-Triassic) of South Dakota has been studied. Results indicate that the amount of nonundulatory extinction, monocrystallinity, and inclusion-free grains in quartz of these rocks is extremely variable. Variability of these characteristics decreases, and the minimum observed values increase, however, with decreasing grain size. Thus, in most thin sections or in grouped results from either formation, populations of successively finer grains (very fine sand to fine silt) contain more nonundulatory, monocrystalline, and inclusion-free grains. Angularity of the quartz grains increases slightly as the size of the grains decreases, but roundness in quartz silt is not related to its internal structure. Sphericity is not related to internal structure or to the size of grains.
Selective abrasion of structurally weak grains may reduce the amounts of undulatory quartz, polycrystalline grains, and quartz with abundant vacuoles and microlites in successive sedimentary cycles. Another possible mechanism is that original size reduction of quartz grains yields increasing amounts of nonundulatory quartz, monocrystalline grains, and inclusion-free grains in finer sizes. We suggest that the second process is probably dominant. There is some evidence to suggest that selective replacement of quartz grains by carbonate cement may reduce the amount of undulatory quartz in successive sedimentary cycles.