ORGANIC ACCUMULATIONS 



171 



transformation takes place much more readily when the CaC0 3 is 

 in the form of aragonite, as is the case in many shells and corals. 

 Mollusca. — The ordinary shell-fish (Mollusca) supply a very 

 large amount of calcareous material for the formation of shallow- 

 water limestones, especially in the neighbourhood of the coasts. 

 The shells accumulate in great banks, frequently, though not always, 

 mingled with more or less sand and mud, and when gathered below 

 the limit of violent wave action, they are entire, embedded in finer 

 material, which is calcareous or not, according to the nature of 











FIG. 69. — Modern shell limestone (Coquina) from Florida. 



the debris swept out from the shore. More commonly the shells 

 are ground by the waves into fragments, making shell sand and 

 mud, which is then cemented into a more or less compact mass. 

 The coquina rock of Florida is an example of a recently made 

 shell limestone (though it is forming no longer), and among the 

 rocks of the earth's crust are many immense limestones which were 

 accumulated in this way. 



Echinodermata. — This group of marine animals, which includes 

 the starfishes, sea-urchins, crinoids or sea-lilies, etc., is made 

 up of forms which all secrete skeletons of calcareous plates, and 

 which contribute largely to the formation of marine limestones. 

 At the present day, however, they seldom build up any extensive 



