Explanations of origin are reviewed and environments in which black muds are in process of deposition are considered. The causes of blackness are examined and attention is given to diagenetic processes that take place in black muds.

It is concluded that colors of black muds are due to several components, of which some are developed in diagenesis and others are due to inclusion of organic matter of various kinds. Sediments that are black at the time of deposition may have the constituents to which blackness is due eliminated during diagenesis, or following lithification. These produce rocks that are not black. It is concluded that black muds develop under several environmental conditions, of which some are found in land waters, some in shallow marine waters, and probably some in deep marine waters, and that retention of blackness is dependent upon the extent of micro-organic processes in the muds.