30 A TEXTBOOK OF OCEANOGRAPHY 



rather than a clayey nature. In some régions they pass 

 gradually into blue and green muds, in others into coral muds 

 and sands, or witli increasing depth into globigerina, pteropod, 

 and diatom oozes, or red clay. There are 38 examples of 

 volcanic muds in the Challenger collections, in depths from 

 260 to 2,800 fathoms, the average being 1,033 fathoms. 



The average percentage of carbonate of lime is 20'5. 



The volcanic muds and sands are found around ail oceanic 

 volcanic islands. They cover an area of about 750,000 square 

 miles. 



Coral Muds and Sands are found in the vicinity of coral 

 reefs and islands. They are derived from the débris of coral 

 reefs. In shallow waters sands are formed, but beyond the 

 limits of wave action a mud of triturated particles of calcareous 

 matter is met with. 



The prédominant feature of the deposit is carbonate of 

 lime, which averages 85 per cent. 



Coral muds and sands cover 2,700,000 square miles of the 

 océan bed. By far the greatest area is in the Pacific (i^ million 

 square miles), the Atlantic coming next with 800,000, and the 

 Indian Océan last with 400,000 square miles. 



Pélagie Deposits and Fauna. 



The pelagic deposits (eupelagic of Kriimmel) are pre- 

 dominantly composed of the shells of marine organisms. The 

 red clay is, however, an exception to this statement. The 

 remains of pelagic organisms that hâve fallen from the surface 

 form the chief part of many of thèse deposits — e.g., the 

 pteropod, globigerina, diatom, and radiolarian oozes. Thèse 

 shells are composed of either carbonate of calcium or silica. 

 In the very deepest areas neither calcareous nor siliceous 

 remains predominate, the basis of the deposit being red clay. 

 This clayey matter is derived from the décomposition of 

 volcanic materials ; quartz or other terrigenous particles are 

 either very rare or entirely absent. The physical conditions 

 in thèse areas are remarkably uniform ; the température is near 



