Several groups of limestones are recognized on the basis of their textural differences. Most of them give evidence of mechanical deposition and they are described as clastic textured. The various types within this clastic-textured group owe their characteristic appearances to the kinds and amounts of four textural components--grains, lime mud (micrite), cement, and void (pores). A nonclastic-textured group of limestones is built up of a fifth textural component, the organic framebuilders. A sixth textural component, recrystallization calcite, modifies the basic limestone types.

Carbonate grains, which are analogous to sand or silt grains, form the rock framework for the mechanically deposited limestones. If grouped together, they are capable of yielding an effectively porous rock. Recognizable carbonate grain types are grouped into five divisions (1) detrital grains--fragments derived from pre-existing rocks, (2) skeletal grains--broken or whole, (3) pellets--grains composed of micritic material, (4) lumps--grain aggregates or composite grains, and (5) coated grains--grains with concentric coating or with rims of calcium carbonate enclosing or encrusting a central nucleus. Important subvarieties of grains are recognized within these gross divisions.

The lime mud or micritic material in a limestone is roughly equivalent to the argillaceous material in a "dirty" sandstone or mudstone. Micritic material refers to unconsolidated or lithified ooze or mud of either chemical or mechanical origin, and is given an arbitrary upper size limit of 0.03 mm. Cement is the clear crystalline component that occupies the interstices between grains and is similar in appearance and distribution to silica cement in sandstones. Void spaces have various shapes and distribution, depending in part on other textural features and in part on genesis. Recrystallization calcite refers to those calcite mosaics resulting from grain enlargement or conversion processes, and excludes calcite cement.

Basic limestone rock types are recognized by (1) noting the types and amounts of grains or framework builders where present, and (2) estimating the relative proportions of grains, framebuilders, and micritic material. A classification based on these objective and measurable features has been developed. These features are used in the rock name. Other features, such as porosity, cementation, grain size, and color, are restricted to the rock description or are used as modifiers before the rock name.

Dolomites must be treated somewhat differently. The system for describing and naming dolomites is based primarily on a compositional grouping into calcareous dolomites and pure dolomites. These groups are modified by appropriate textural terms.