The most important problem in the interpretation of the micropetrology of sandstones is the evaluation of the abundance of detritus of first-cycle as opposed to polycyclic origin. At present few tools or data with which to make this distinction exist, largely because of the lack of study by sedimentary petrologists of igneous rocks, metamorphic rocks, and soils. Also, studies of relative rates of abrasion of sand-sized rock fragments and feldspar are rare. Most igneous and metamorphic rocks are equilibrium or near equilibrium assemblages of minerals. The mineral species present in such rocks are controlled by the laws of physical chemistry applied to the bulk chemical composition of the magma or pre-existing rocks. In contrast, sandstones are invariably non-equilibrium mineral assemblages and, therefore, the mineralogic character and origin of each sand grain must be considered as independent of surrounding grains. For this reason sedimentary petrology is a more difficult field of study than igneous or metamorphic petrology. The interpretation of ultimate provenances of sandstones has suffered from over-application of generalizations and under-application of detailed mineralogic analyses and interpretations. New ways of analyzing data concerning the mineral composition of sandstones are needed.

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