ACCUMULATION IN SHALLOW WATER 165 



distances from its place of origin. An instance of this is the 

 Atlantic coast of Florida, which has a siliceous sand belt that 

 cannot have been derived from the peninsula. 



Organic deposits are much less common in shallow water than 

 are the terrigenous, and yet under favourable conditions they are 

 developed on a very extensive scale. The most important of such 

 conditions are warm water and the presence of ocean currents 

 which bring abundant supplies of food. The sea is constantly 

 receiving from the land materials in solution, of which the most 

 important are the carbonate and sulphate of lime. Many classes 

 of marine animals extract the CaC0 3 from the sea-water and form 

 it into hard parts, either as external shells and tests, or as internal 

 skeletons. There is also good reason to believe that some, at 

 least, of these organisms are able to convert the sulphate into the 

 carbonate. 



The classes of marine organisms which at present or in times 

 past have played the most important part in the accumulation of 

 calcareous material are : the Foraminifera, Corals, Echinoderms, 

 and Molluscs ; but other groups contribute extensively to the same 

 result. The Foraminifera do not accumulate with sufficient rapid- 

 ity to add largely to the calcareous deposits of shallow water, and 

 will therefore be considered in connection with the deep-sea for- 

 mations. 



Corals. — The animals of this group are of many varieties of 

 form, size, and habit, and by no means all of them are important 

 as rock-makers. The solitary corals, for example, are widely dis- 

 tributed in the deep sea, but are never sufficiently abundant to 

 form deposits by themselves". Those corals which do accumulate 

 in great masses, and are known as reef-builders, form compound 

 colonies or stocks, in which hundreds or thousands of individuals 

 are united. The adult corals are stationary, but the newly hatched 

 young are worm-like, free-swimming larvae. When the young ani- 

 mal establishes itself in a suitable place, it develops into a polyp, or 

 fleshy sac, with a mouth surrounded by rows of tentacles, and then 

 by budding or partial division (fission) gives rise to great numbers 

 of other polyps, which are connected together by a tissue common 



