The 100-fathom line is by Murray and Renard regarded as the boundary between shallow and deep water, for it generally marks the edge of the continental shelf, from which the bottom rises very gently to the land, but slopes abruptly down to the oceanic depression. The great bulk of the material derived from the waste of the land is thrown down upon the continental shelf, within the 100-fathom line, but the liner particles are carried farther out and subside in deeper and quieter water. A considerable quantity of the finest sedimentary particles remains long suspended in sea-water, especially in the cold water of the polar seas. On the continental slopes, extending from the 100-fathom line to the bottom of the great oceanic abysses, are laid down most of the very fine materials derived from the land, which are grouped together under the somewhat indefinite term, mud. Mud is a mixture of minerals in a state of extremely fine mechanical subdivision, but not chemically decomposed, thus differing from clay.

(1) Blue Mud

The materials of this deposit, which are principally, though not altogether, derived from the land, are very heterogeneous. Quartz grains in an excessively fine state of subdivision are very abundant; clay is often a considerable ingredient, and then the mud is plastic when wet, but it is usually more earthy than clay-like. Minute particles of other terrigenous minerals, like felspar, hornblende, augite, etc., are common. CaC03 is almost always present, averaging 7%, and in some instances rising to 25 %; this is due chiefly to the foraminiferal shells, both of those species which live at the surface and those which live on the bottom. Siliceous organisms are also present to the average amount of 3 %, and are principally diatoms, radiolarians, and spicules of sponges. Glauconite is found in nearly all the samples. The blue colour of this mud is due to the sulphide of iron and the organic matter which prevents the oxidation of the sulphide. Of the terrigenous deep-sea deposits blue mud is the most extensively developed; it is estimated as covering 14,500,000 square miles of the sea-bottom, and surrounds almost all coasts, and fills enclosed basins like the Mediterranean and even the Arctic Ocean. The depths at which blue mud is found range from 125 to 2800 fathoms.

Fig. 141. - Map of marine deposits. (Kayser after Murray and Renard) Dotted area = terrigenous; vertical lined = foraminiferal ooze; horizontal broken-lined = diatom ooze; crosses = oceanic red clay; white = radiolarian ooze.

(2) Red Mud

Red Mud is a local development, which occurs principally upon the Atlantic coast of Brazil, and in the Yellow Sea of China. Silt of this character, the red colour of which is due to Fe203, contained in laterite, is brought down in large quantities by the Amazon and the Orinoco. Foraminiferal shells are abundant; radio-larians very rare. Probably a more minute examination of the continental slopes will show that red mud has a wider distribution in tropical seas than is here indicated.

(3) Green Mud

Green Mud is much the same in character as the blue mud, but owes its green colour to the higher percentage of glauconite which it contains.

(4) Green Sand

Green Sand is granular in appearance, and is made up largely of grains of glauconite and casts in that material of the interior of foraminiferal shells, together with nearly 50 % of CaC03. The green sands occur in shallower water than the muds, and often within the 100-fathom line, as in the case of a deposit of this kind which is now forming off the coast of Georgia and the Carolinas. The estimated area of the green muds and sands is 1,000,000 square miles.

(5) Volcanic Muds

In the deeper water surrounding volcanic islands are deposits of fine mud made from the disintegration of volcanic rocks, mixed with considerable clay, and also calcareous matter derived from organisms.