DOLOMITIZATION AFTER LIMESTONE EMERGES FROM SEA. 805 



explained on pages 655-656, 764-766, that transfer of material and change 

 in chemical composition are extensive and great in the zone of katamor- 

 phism and are trivial and small in the zone of anamorphism. Further, in 

 the zone of katamorphism it is believed that dolomitization mainly occurs 

 in the belt of cementation rather than in the belt of weathering, although 

 the process undoubtedly takes place in both belts. 



We have seen that rocks contain magnesium mainly as a silicate or 

 carbonate. If above the level of ground water the magnesium be as silicate, 

 the process of carbonation changes it to a carbonate or other compound, 

 and as such it is taken into solution and carried downward to the sea of 

 ground waters. If the magnesium be as carbonate, the process of carbona- 

 tion is not necessary. In any case the waters of the belt of weathering 

 steadily carry downward magnesium and calcium carbonate and other salts 

 to the belt of cementation. These solutions pass into limestones and other 

 formations. 



When the solutions pass into limestones which contain little magnesium, 

 the process of replacement begins. The substitution of magnesium for cal- 

 cium is controlled by various factors, among which time, the absolute and 

 relative amounts of" calcium and magnesium in the solutions and in the 

 rocks, the change in volume in consequence of the reactions, and the 

 direction of the movement of the solutions are important. 



It is certain that time is an important factor. The underground 

 solutions are continually circulating, and there are ever-renewed supplies 

 of magnesium to carry on the process in the solutions which join the belt 

 of cementation. At the outset calcium is greatly predominant in the 

 limestone ; but as the substitution goes on, magnesium becomes more and 

 more important, and may finally become molecularly equivalent to cal- 

 cium. But to accomplish this must require much time, for the solutions 

 at any moment occupy but a small percentage of the volume of the rock, 

 and they contain but a very small amount of magnesium. Therefore the 

 magnesium held at any one time by the solutions is but an inappreciable 

 fraction of the total amount which is finally introduced into the rock. 



The absolute and relative amounts of calcium and magnesium in the 

 solutions and the limestones is a matter of the greatest consequence in 

 dolomitization. The law of mass action requires that where solid calcium 

 carbonate is in contact with magnesium- and calcium-bearing solutions a 



