Methods are given for computing the gross mineral compositions of sedimentary rocks. The methods are based on the use of simultaneous equations which are written to express the distribution of Al 2 O 3 and K 2 O among various mineral constituents, principally clay and potassium feldspar. This approach has been used in studies of sedimentary rocks and sandstone-type U ores on the Colorado Plateau in which clay and potassium feldspar are the principal aluminous and potassic constituents. The methods may be modified for use in studying sedimentary materials in which other aluminous and potassic constituents are present. If the composition of the clay in the rocks which are being studied is largely unknown, the mineral compositions of the rocks may not be computed precisely. However, if the composition of the clay is known within limits it is possible to compute ranges of concentration for each mineral constituent. The ranges are broad where the rocks contain abundant clay of little-known composition, but are less broad where the rock is clay-poor or where the approximate composition of the clay can be presumed. It is not necessary to consider the complete composition of the clay to compute the gross mineral composition of the rock. The methods may also be used to interpret some chemical characteristics of the clay in the rocks being studied from certain aspects of their known mineral and chemical composition.