CORALS AND CORAL REEFS — VAUGHAN. 217 



arrangement of the material in the Bahamian ridges and of that 

 in a sand dune at Cape Henry. The sand at Cape Henry is siliceous 

 (quartz) sand; while that composing the hills and ridges in the 

 Bahamas is calcareous, almost pure, more than 99 per cent, car- 

 bonate of lime. Limestone composed of grains similar to the grains 

 in the wind-formed hills underlies the surface of the low, flat areas 

 in the Bahamas, but its grains have not been wind-blown. They 

 were formed in the sea and were later uplifted so that they now 

 stand above sea-level. As this kind of limestone has been improp- 

 erly called coral rock, a short account of the mode of its formation 

 will be given. 



A close inspection of a piece of this rock, even with the naked 

 eye, reveals that it is composed of minute balls and ovoid or ellip- 

 soid bodies, from 0.2 to about 1 millimeter in diameter, set into a 

 cementing groundmass. Plate 31, figrre A, illustrates the surface 

 of a specimen natural size, and figure B represents a part of the 

 same surface enlarged 10 times. Because the ball-like bodies com- 

 posing the rock give it an appearance similar to fish roe, it is 

 known as oolite, which means egg rock. Plate 32, figure 1 illus- 

 trates a thin slice of a single grain magnified 100 times. It is en- 

 tirely obvious that these bodies are composed of concentric coats, 

 and that they were formed by some process that caused outer coats 

 to be successively laid down on the inner ones. It was stated in 

 the preceding paragraph that this rock contains more than 99 pei 

 cent calcium carbonate, and that the egglike granules originated in 

 the sea. How was the carbonate of lime taken out of the sea? 



Recent investigations have very clearly shown that there is in the 

 shallow waters of the tropical and subtropical parts of the ocean as 

 much carbonate of lime in solution as it is possible for the water to 

 hold — in other words, the water is saturated with carbonate of lime. 

 It is therefore clear that any agency that will reduce the capacity 

 of such water already saturated to hold calcium carbonate in solu- 

 tion will cause that substance to be precipitated. The principal 

 solvent of calcium carbonate in sea water is carbon dioxide (C0 2 ), 

 popularly known as carbonic-acid gas, and the reduction of the 

 amount of it in the sea water will produce precipitation. Raising 

 the temperature of the water, whether naturally or artificially, re- 

 duces its capacity to hold C0 2 , and agitation, if there is too little 

 CO, in the air, will hasten the process. Evaporation, leading to a 

 greater concentration of salts in the water, will also cause precipi- 

 tation of calcium carbonate. 



Besides the inorganic agencies mentioned, there are organic 

 agencies that cause the precipitation of calcium carbonate in the 

 sea. It has been known for a long time that the addition of a strong 

 alkali, such as ammonia, to sea water will produce precipitation of 



