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MARINE DEPOSITS 



masses unassisted, but in former ages of the world's history they 

 did so on a great scale. This is particularly true of the crinoids 

 (sea-lilies or feather-stars), which have now become comparatively 

 rare, but many ancient limestones are composed almost entirely of 

 their remains, and especially of their hard and heavy stems. 



FlG. 70. — Ancient limestone composed of various kinds of organisms. 



Limestone Banks. — In favourable situations immense submarine 

 plateaus or banks are built up in shallow waters by the accumulated 

 remains of all sorts of lime-secreting animals, corals, echinoderms, 

 molluscs, worms, and Foraminifera. These are well exemplified in 

 the Gulf of Mexico and the Caribbean Sea by the great banks along 

 the west coast of Florida, the Yucatan bank, and the plateau which 

 extends from the coast of Nicaragua almost to Jamaica. On these 



