Three types of mottling are conspicuous in the dolomites and limestones of the Simonson formation, middle Devonian age of eastern Nevada. Petrographic and chemical studies combined with field observations indicate probable origins of these types. A review of other reports of mottled carbonate rocks indicates the existence of several additional types of mottling. Each type is the result of variations of the processes which cause the segregation of the component minerals. These processes may differ during deposition of the sediments or at any later time. A genetic classification is proposed which may be useful for interpreting the depositional environment and diagenetic history of the various types of mottled carbonates. The major groups of the classification are: organic control, depositional control, intrastratal deformational control, diagenetic control, and depositional and diagenetic control.