Very fine grained sedimentary rocks are defined as those containing more than 45% by volume microscopic material (<5 μm). Those containing 65% to 45% microscopic material are designated as mudstones, and those containing more than 65% microscopic material are designated as shales. The increase of detailed studies on these rock types has revealed the need of a more descriptive and laboratory-oriented classification for them. The proposed classification is based on textural and compositional attributes of the rocks. The nomenclature consists of a root name preceded by a primary adjective, which, in turn, may be preceded by a nominal adjective. The root names include claystone, marlstone, micstone, and mudstone and are defined on the basis of volume percentage of microscopic material and weight percentage of silicate minerals in the rock. Primary adjectives convey specific mineralogic information about the rock; nominal adjectives may be used to emphasize other attributes, such as bedding structures, fissility, color, or fossil content.