204 The Realm of Nature CHAP. 



creatures living in the water where it is deposited, but it 

 accumulates so rapidly that shells as a rule form a very 

 small proportion of the whole. 



271. Green Mud. Along cliff- bound coasts in which 

 few rivers open, terrigenous deposits form very slowly, and 

 to a distance only of 100 miles, or less, from land. The 

 finely-ground particles of rocks are thus exposed for a long 

 time to the action of sea- water and undergo extensive 

 chemical changes. A greenish mineral called Glanconite 

 is thus produced, which fills up the interior of dead calcar- 

 eous shells, forming casts of the interior which remain when 

 the shells themselves are dissolved away by weak acids. 



272. Volcanic and Coral Muds and Sands. Oceanic 

 islands of volcanic origin are surrounded by Volcanic Muds 

 or Sands, formed by the wearing down of volcanic rock and 

 its subsequent partial decomposition by the chemical action 

 of sea-water, the fragments of shells which are present being 

 often coated with peroxide of manganese derived from the 

 rocks. Islands of Coral origin are in a similar way sur- 

 rounded by Coral Muds or Sands which consist almost 

 entirely of carbonate of lime. The remains of calcareous 

 marine plants (chiefly corallines) often make up a large part 

 of this deposit. 



273. Siliceous and Calcareous Organisms. Certain 

 minute moving organisms or living creatures, rarely visible 

 except by means of the microscope, and possessed of the 

 power of secreting silica from solution in sea-water, are 

 found in the surface layers of all oceans, especially where the 

 salinity is slight. One kind, known as Diatoms, abounds in 

 cold seas and in estuaries, forming delicate cases or shells 

 exquisitely marked. They probably obtain some of their 

 silica by decomposing the clayey mud of rivers. Radio- 

 larians, another class of silica-secreting organisms, frequent 

 warmer water and are not found in estuaries ; they form 

 a minute framework or skeleton of glassy spicules often 

 arranged in very complex and beautiful groups. The 

 chief pelagic molluscs living on the surface far from land 

 are a few kinds called Heteropods and Pteropods and they 

 inhabit tropical seas. Their shells are thin papery cases 



