l62 



DEPTHS OF THE OCEAN 



Terrigenous 

 deposits. 



Blue mud. 



Green mud 



and sand. 



Red mud. 



Volcanic mud 

 and sand. 



Coral mud 

 and sand. 



Pelagic 

 deposits. 



The Terrigenous Deposits are characterised, as already- 

 stated, by the abundance of land-detritus, and are subdivided 

 into the following types, viz. : — 



Blue Mud. — This is the predominant type of deposit in the 

 neighbourhood of continental land, and is principally made up 

 of land-detritus (quartz being the characteristic mineral species), 

 which becomes less and less abundant with increasing distance 

 from the land, until the Blue mud passes gradually into one of 

 the types of pelagic deposits. 



Green Mud is a variety of Blue mud, distinguished by the 

 abundance of grains of glauconite usually associated with 

 phosphatic concretions, and is found most characteristically on 

 the continental slopes off high and bold coasts where currents 

 from different sources alternate with the season, as off the 

 Cape of Good Hope, off the east coast of Australia, off Japan, 

 and off the Atlantic coasts of the United States. In the lesser 

 depths the amount of clayey and muddy matter decreases and 

 the deposits are called Green Sands. 



Red Mud is a local variety of Blue mud found in the Yellow 

 Sea and off the coast of Brazil, where the great rivers bring 

 down a large amount of ochreous matter, to which the deposit 

 owes its colour and its name. 



Volcanic Mud occurs off those coasts and islands where 

 volcanic rocks prevail ; the volcanic mineral particles are larger 

 and more abundant in the shallower water near the land, and 

 the deposits there are called Volcanic Sands. 



Coral Mud is found in the vicinity of coral reefs and islands ; 

 fragments derived from the disintegration of the reefs are 

 larger and intermixed with less fine material in the lesser 

 depths, and the deposits are then called Coral Sands. 



The Pelagic Deposits are characterised by the fact that, 

 with the exception of Red clay, their composition is largely 

 determined by the pelagic or plankton organisms, which secrete 

 hard shells either of calcium carbonate or of silica, the pre- 

 dominance of the remains of one or other of these classes of 

 organisms giving the names to the deposits. In fact, the 

 deposits may be divided into those that are calcareous and 

 those that are siliceous, the calcareous deposits (Globigerina 

 ooze and Pteropod ooze) being characteristic of tropical and 

 subtropical regions, where there is abundant secretion of calcium 

 carbonate by plankton organisms, the siliceous deposits (Diatom 

 ooze and Radiolarian ooze) being characteristic of polar and 

 other regions, where there is a large admixture of clayey matter 



