28 A TEXTBOOK OF OCEANOGRAPHY 



the merest trace in 2,650 fathoms to 35 per cent, in 500 fathoms. 

 The pelagic foraminifera make up 7^ per cent, of the CaCO 3 

 The blue muds surround nearly all coasts and fill nearly all 

 enclosed seas e.g., the Mediterranean and the Arctic. Of 

 all the terrigenous deposits they occupy the greatest space 

 viz., 14^ million square miles, of which 4 millions are in the 

 Arctic, 3 in the Pacific, 2 in the Atlantic, ij in the Indian 

 Ocean . 



Since the Challenger expedition Weber has collected 

 typical blue mud in the Siboga, especially in Banda Sea. 



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

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

 sediment from the land. It is characteristic of tropical and 

 subtropical seas into which drain rivers traversing laterite* 

 areas. 



The red colour is due to large quantities of ochreous 

 matters, the colouring being due to iron oxide. Red mud is 

 especially characteristic pf the South American shelf. It 

 contains no glauconite. The organic matters present, though 

 abundant, are not sufficient to reduce the peroxide of iron to 

 the state of protoxide. 



Murray and Renard describe ten samples from the 

 Challenger expedition, the percentage of calcium carbonate 

 ranging from 6 to 61, the average being 32. This car- 

 bonate of lime is derived from the shells of pelagic 

 foraminifera. 



Siliceous organisms such as diatoms and radiolaria are very 

 rare. The mineral particles from the neighbouring land con- 

 stitute from 10 to 25 per cent, of the whole, the average being 

 21 per cent. 



Green Muds and Sands. Green mud is a variety of blue 

 mud distinguished by an abundance of grains of glauconite, 

 usually associated with phosphatic concretions. It is found 

 off high coasts with few rivers to pour detritus into the sea 



* Laterite is a cellular, reddish, ferruginous clay found in some tropical 

 countries as the result of subaerial decomposition of rocks. 



