792 A TREATISE ON METAMORPHISM. 



I'K'iANir I'KECIPITATF.S. 



Animals of the ocean and lakes abstract calcium carbonate from solution 

 and build it into their hard parts, The number of kinds of organisms which 

 are doing this work is great, Of these the more important classes are the 

 .corals, mollusks, and crinoids, the relative importance probably being 

 in the order given. In each class are many species, and the number of 

 individuals of each species is beyond computation. In order that a great 

 quantity of material may be abstracted from the water by organisms it is 

 necessary that abundant material shall be furnished to it, and also that the 

 conditions shall be favorable for the existence of abundant animal life of 

 the right kind. The material for the building of limestone is furnished 

 to the ocean mainly by the streams. Therefore, adjacent to great bodies 

 of land from which large rivers enter into the sea there is a continuous 

 and abundant supply of calcium carbonate. Sluggish rivers which have 

 meandered through limestone regions furnish an especially abundant 

 supply. An essential condition for abundant animal life in the sea is a 

 warm climate, and hence the limestones are likely to be formed most 

 plentifully in the tropical or subtropical regions. Tropical and subtropical 

 regions are favorable for limestone building for the further reason that 

 evaporation is great and concentration of calcium carbonate is thus pro- 

 duced. This is of great importance in shallow, nearly inclosed seas, and 

 lagoons. The areas which combine the two conditions — abundance of 

 material and abundance of life — are those in which the great limestone 

 building of the present time is going on. Such regions are illustrated by 

 Florida and the Bahamas, the sea adjacent to which is constantly fed by 

 the currents from the Gulf of Mexico, which receives the great contri- 

 butions of calcium carbonate of the Mississippi and many other rivers; and 

 by the regions adjacent to the Yellow Sea, which receives the contributions 

 of the Yangtze, Hoangho, and many other important rivers. But the 

 immediate contributions of great rivers are not necessary for extensive 

 limestone formations, for the ocean currents distribute material throughout 

 the seas, and the water contains a sufficient amount of calcium carbonate, 

 so that where other conditions are favorable extensive limestone formations 

 may be built. This is illustrated by the Great Barrier Reef of Australia 

 and the numerous coral reefs about the tropical islands of the Pacific. 



