The carbon cycle is described as a series of biological and geological processes comprising two phases: a degradative and an accumulative phase. A distinction is drawn between environments in which aerobic microbiological changes proceed at a rapid rate, as in soil, and anaerobic environments in which processes of decomposition are at a minimum, as in shallow basins such as swamps and bogs. It is suggested that there exists a spectrum of organic decay in which the aerobic processes are at one extreme while anaerobic conditions resulting in organic accumulation are at the other extreme. The degree of change and the duration of the change vary profoundly throughout this spectrum. Various degrees of decomposition are illustrated with examples of woody materials whose environments of deposition are known and, for the more recent material, whose ages were determined by the carbon isotope method. Photomicrographs and detailed descriptive data are presented.