Red mud may be classed as a variety of blue mud, from which it differs on account of the larger proportion of ochreous sub stance. This variety surrounds the tropical parts of the conti nental shelves of South America, South Africa and eastern China.
Green mud differs to a greater extent from the blue mud, and owes its characteristic nature and colour to the presence of glauconite, the spines of echini and the spicules of sponges. It occurs in such abundance in certain geological formations as to give rise to the name of green-sand. Green mud abounds off Cape Hatteras, the north of Cuba, and off the coast of California. The "Challenger" expedition found it on the Agulhas bank, on the eastern coasts of Australia, Japan, South America and on the west coast of Portugal. When the proportion of calcium carbonate in the blue mud is considerable a calcareous ooze results. The floors of the Caribbean, Cayman and Mexican basins in the Central American sea are covered with a white calcareous ooze with cal careous concretions. Similar formations are found in the Medi terranean, where a dark mud predominates in the western part, passing into a grey, marly slime in the Tyrrhenian basin and re placed by a calcareous ooze in the Eastern basin. The bottom of the Black sea is covered by a stiff blue mud. Sir John Murray found pyrites make up nearly so% of the deposit. In the Red sea the "Pola" expedition discovered a calcareous ooze similar to that of the Mediterranean, and the formation of stony crust by precipitation of calcium and magnesium carbonates may be rec ognized as giving origin to a recent dolomite.
The terrigenous ingredients in the deposits become less and less abundant as one goes farther into the deep ocean. Finely-divided colloidal clay is found in all parts of the ocean, however remote from land, in very small amount, and there is less in tropical than in cooler waters. There is present in sea-water far from land a different clay derived from the decomposition of volcanic materials. To this very slowly-growing deposit of inorganic material over the ocean floor there is added an overwhelmingly more rapid contribution of the remains of calcareous and siliceous planktonic and benthonic organisms, which tend to bury the slower accumulating material under a blanket of globigerina, pteropod, diatom or radiolarian ooze. When those deposits of organic origin are wanting or have been removed, the red clay composed of the mineral constituents is found alone. It is a re markable geographical fact that on the rises and in the basins of moderate depth of the open ocean the organic oozes preponderate, but in the abysmal depressions below 5,000 metres there is found only the red clay, with a minimum of calcium carbonate, though sometimes with a considerable admixture of the siliceous remains of radiolarians.