IV DEPTHS AND DEPOSITS OF THE OCEAN 159 



Associated with the glauconite in certain localities, more Phosphatic 

 especially off the Cape of Good Hope and off the Atlantic coast '^°"'^'^^ti°"^- 

 of the United States, irregular concretions, largely made up of 

 phosphate of lime, have been dredged. The concretions vary 

 greatly in size and form, with a greenish or brownish glazed 

 external surface, and are made up of 

 heterogeneous fragments derived from 

 the deposit containing the concretions 

 (grains of glauconite and other minerals 

 or remains of organisms), cemented 

 by phosphatic material, which consti- 

 tutes the principal part of the concre- 

 tions. When the cemented particles 

 are purely mineral, the phosphatic 

 matter acts simply as a cement, but 

 when the remains of calcareous organ- 

 isms are included in the concretions, 

 the phosphatic material plays a more 

 important part, filling the internal 

 chambers, and often the calcium car- 

 bonate of the shell is pseudomor- 

 phosed into calcium phosphate. When 

 the filling up of a foraminifer, for 

 example, and the pseudomorphism of 

 its shell, are complete, the phosphate, 

 attracted around this little centre con- 

 tinues to be added at the surface, and 

 thus a phosphatic granule is formed, 

 the external appearance of which no 

 longer recalls that of the organism 

 around which the phosphate has 

 grouped itself. These phosphatic con- 

 cretions occur chiefly along coasts 

 bathed by waters subject at times to 

 great and rapid changes of tempera- 

 ture, which cause the destruction on a 



large scale of marine life, the decomposition of the organic 

 remains, sometimes thickly covering the sea-floor in such locali- 

 ties, giving rise to the phosphate of lime to be permanently 

 fixed in the phosphatic nodules. 



Just as the silicate glauconite occurs in the terrigenous Phiiiipsite. 

 deposits, and is supposed to be a secondary product derived 

 from the decomposition of continental rock fragments, so the 



Fig. 134. — Manganese Nodule 

 with scalpellvm darwinil 

 growing on it. 

 " Challenger" Station 299, South 

 Pacific, 2160 fathoms. 



