Classification of limestones should consider (1) mode of origin of CaCO₃ (chemical [biochemical or physicochemical] and mechanical), (2) form of CaCO₃ (skeletal [secretionary] or nonskeletal [accretionary or particulate]), and (3) the processes of deposition and accumulation of units of limestone. The biological and other genetic aspects of limestones are interrelated but commonly difficult to evaluate. Studies of modern carbonate sediments demonstrate that organisms play a dominant role in the formation of skeletal and nonskeletal material. Disintegration of skeletal material to fine sand and smaller size generally renders such material unrecognizable regarding its skeletal nature and biological origin. Nonskeletal carbonate sediments of biological rigin resemble, and are difficult to distinguish from, carbonates of physico-chemical origin.

Progressive lithification and diagenetic changes with increasing age of the carbonate rocks compound the problems of identifying the nature and origin of carbonate particles. The result is a decrease in the uniformity and accuracy of description and classification of carbonate sediments.

The varied concepts of classification of carbonate sediments have resulted from variation in background, interest, methods employed, and purpose of an investigation. The geographic extent and geologic range of the investigation likewise impart a control to the problem of each limestone classification. Ultimately, the classification of carbonate rocks will have to provide for the needs of the field geologist, subsurface geologist, paleontologist, petrographer, and geochemist. Each of these fields of investigation is concerned with only a part of the total evidence available to be used in classification of carbonate rocks. A three-stage procedure of description of carbonate rocks, each with its applicable terminology, is necessary before all descriptions and purposes concerned with carb nate rocks can be satisfied. These stages consist of (1) field description of limestone units, (2) low-power binocular description of samples, and (3) thin-section and geochemical description of samples. Most descriptive data in the procedure are ultimately based on field observations. Generally, it is only after all three stages have been fulfilled that origin can be assigned to ancient carbonate rocks.