222 THE OCEAN 



calcareous organisms, and revealed in the 

 deposits after treatment with dilute acid as 

 greenish casts and rounded green grains, some- 

 times in such abundance that the deposits 

 are called green muds or green sands. 



Barium Nodules. — Small quantities of sul- 

 phate of barium have been detected in many 

 marine deposits, and off the coast of India 

 small spherical nodules containing 75 per cent, 

 of barium sulphate have been dredged in 675 

 fathoms of water. 



Manganese Nodules. — The peroxides of man- 

 ganese and iron in the form of small grains 

 are widely distributed throughout all deep- 

 sea deposits, and nodules of various sizes occur 

 in great abundance in certain red clay areas, 

 especially of the Central Pacific, concreted 

 around various nuclei such as sharks' teeth, 

 cetacean earbones, fragments of volcanic 

 rocks and glasses, pumice, etc. 



Teeth of Sharks and Earbones of Whales. — 

 Sharks' teeth are dredged in considerable 

 numbers sometimes in very deep water, 

 more especially in the Central Pacific, but it 

 is merely the external shell (the hard dentine 

 or enamel) that is preserved, the internal 

 portion and base having been removed. These 

 teeth are all impregnated and more or less 

 thickly covered by manganese peroxide. The 

 dense earbones and beaks of whales have been 



