METAMORPHISM OF CALCIUM CARBONATE DEPOSITS. 795 



organic precipitation must have more than equaled the additions of calcium 

 carbonate from the land, and thus have reduced the total amount in solution. 

 In the present state of knowledge it must be said that the precipitation 

 by chemical means of great limestone formations in the ocean, or in medi- 

 terranean seas connected with the ocean, is a speculation based upon the 

 premised inadequacy of life in early times to precipitate such amounts of 

 materials, and is required to explain only the limestones of pre-Paleozoic 

 times, the rocks of which contain very scanty fossils. But even these lime- 

 stones may have been precipitated by organisms and the fossils destroyed 

 by the processes of recrystallization and metamorphism subsequently 

 described, just as they have been in many later formations. 



METAMORPHISM OF ORGANIC AND CHEMICAL CALCIUM CARBONATE DEPOSITS. 



During the time that the various forces and agents are abstracting the 

 calcium carbonate from the sea the mechanical and chemical forces are at 

 work. No sooner is a deposit formed than the waves may break it into 

 detritus. All the organic and chemical deposits within the reach of the 

 waves and currents are handled by them. Thus the material may be 

 widely distributed according to the laws of the distribution of mechanical 

 deposits. (See Chapter VI, pp. 555-560.) To illustrate: The de'bris of 

 the coral reefs is carried seaward until the water reaches such a depth that 

 the undertow has lost its power. Thus conglomerate, sand, and silt com- 

 posed of limestone may be deposited in concentric belts. (PL IV, A.) 

 Any of the shore deposits described by Gilbert may be built up. a But 

 the mechanical work does not go on alone. The moment an animal dies the 

 calcium carbonate is subjected to the action of solution and deposition by 

 the sea. When the material is exposed to the open sea, or when there are 

 continuous currents, the water is not saturated with calcium carbonate, and 

 there is continual solution of the material at the rock surface. Within the 

 body of the rock the particles may be cemented by deposition. (PI. IV, B.) 

 In inclosed or nearly inclosed areas, as, for instance, locally between the 

 islands and reefs encircling Australia, the evaporation may more than compen- 

 sate for the influx of fresh water, and thus the tendency, on the whole, be to 

 precipitate additional material from the sea, rather than to dissolve material. 



At the same time limestone is forming there may be a substitution of 

 magnesium for calcium. The processes of solution and redeposition and of 



"Gilbert, G. K., Lake Bonneville: Mon. U. S. Geol. Survey, vol. 1, 1890, pp. 23-89. 



