The water in enclosed basins that have shallow outlets to the sea, such as some Norwegian fjords, tends to stagnate. Salt water entering the fjords from the ocean, due to its relatively great density, sinks to the bottom. The river water being lighter, floats on top. The salt water is thus trapped and gradually loses its supply of oxygen, unless it is ventilated, that is unless heavier water enters the fjord from the sea and sinks to the bottom, thus displacing upward the stagnating bottom water. If the bottom water is not ventilated, stagnation continues until all the oxygen is consumed and then hydrogen sulphide begins to form. This water that contains H₂S acts as a fairly effective preservative for the remains of organic matter that fall into it, with the re ult that black deposits having a high organic content are formed. Other areas of stagnant water in which black deposits are being deposited are the Black Sea and numerous lakes in the tropics. At the present time, the water at the bottom of most marine basins contains some oxygen, with the result that black deposits similar to those in some of the Norwegian fjords are accumulating today in relatively few places in the sea. In the past, however, conditions in the ocean may have been different and H₂S may have formed in the bottom of some basins. In this way, black shales, such as are commonly found in early Paleozoic deposits, may have been deposited. (Editor's abstract.)