DOLOMITIZATION BEFORE LIMESTONE EMERGES FROM SEA. 803 



system (see Chapter III, pp. 90-91), the sea not being saturated with 

 either magnesium or calcium salts, but containing more than three times as 

 much magnesium as calcium, and having below it in various regions solid 

 calcium carbonate. Under these circumstances there is a tendency for some 

 of the calcium of the solid carbonate to change places with the magnesium 

 in solution, and thus dolomitization may take place beneath the sea. 



If the sea were saturated with magnesium salts it is probable that the 

 replacement would go on rapidly. Since, however, the sea is far from sat- 

 urated with such compounds, and since, as already shown (pp. 798-799), it 

 can not be supposed that the sea has ever been richer in magnesium salts 

 than at present, the change has taken place slowly. But the substitution 

 is continuous; for, in consequence of the currents, the water depleted in 

 magnesium is replaced by water containing a normal amount of that 

 element. But the solutions of the ocean also contain calcium. Therefore 

 the conditions are approximately as if one ran magnesium-calcium solutions 

 like those of the ocean throug-h solid calcium carbonate until equilibrium 

 ■under the laws of chemistry (see Chapter III, p. 90) had been reached. 

 But in the ocean equilibrium is probably but rarely reached on an extensive 

 scale because of the slow and imperfect manner in which the solutions make 

 their way through the calcium-carbonate deposits at the bottom of the sea. 

 Occasionally in the arms or shut-off jiortions of the sea, where the conditions 

 are favorable for unusual concentration, the amount of magnesium in the 

 water may be even greater than that in the normal waters, and under these 

 circumstances the reaction would go on more rapidly. This is illustrated 

 by the coral reefs of Metia, which are heavily magnesiah. This case 

 furnishes positive evidence that dolomitization may locally, under favorable 

 circumstances, go very far while a limestone is below the sea. Dana's 

 explanation of the unusual amount of magnesium is that "the sand or mud 

 may have been that of a contracting and evaporating lagoon, in which 

 magnesian and other salts of the ocean were in a concentrated state."* 

 Dana compares concentration of magnesium in lagoons mainly shut off 

 from the sea to concentration in salt pans. 6 



While it is clear that dolomitization below the sea may locally go 

 far, the usual facts of observation correspond with the conclusion above 



«Dana, Jas. D., Corals and coral islands, New York, Dodd, Mead & Co., 3d ed., 1890, p. 394. 

 6 Dana, Jas. D., Manual of geology, American Book Company, 4th ed., 1895, p. 133. 



