802 A TREATISE ON METAMORPHISM. 



concius IO n. — From the foregoing facts it is concluded that the major part 

 of the magnesium in dolomitic limestones and dolomites is due to replace- 

 ment. In the instances of rapid local variation in the amount of magnesium 

 in the limestones of coral islands and in the limestones of England, Ireland, 

 and America, where the amount of magnesium is greatest near the surface 

 or adjacent to fractures, there seems to be no other conclusion. And there 

 is no reason to doubt that the magnesian character of the great dolomite 

 formations is due to replacement. 



HOW AND WHY DOLOMITIZATION OCCUKS. 



The magnesium for dolomitization is primarily derived from magnesium- 

 bearing minerals of the original igneous rocks. The more important of 

 these are the micas, pyroxenes, amphiboles, and olivines. But at the 

 present time a large part of the magnesium for dolomitization is derived from 

 secondary minerals, such as garnet, staurolite, tourmaline, chondrodite, 

 chlorite, the zeolites, and even from previously formed dolomite. Indeed, 

 the magnesium concentrated in earlier dolomite is one of the most impor- 

 tant sources for present dolomitization. From these various sources the 

 processes of alteration of the zone of katamorphism, and especially of 

 the belt of weathering, produce soluble magnesium compounds, such as 

 carbonate, hydrate, sulphate, etc. The magnesium compounds formed or 

 dissolved in the belt of weathering are transported in part to the belt of 

 cementation and in part to the sea, either before or after an underground 

 journey. This process has been continuous since the continents were 

 divided from the seas, consequently the amount of magnesium in the sea has 

 probably continuously increased since early geological times, for it can not 

 be supposed that, on the average, more magnesium has been precipitated 

 from the sea than has been added to it during any geological period. It is 

 seen in Chapter X that the amount of magnesium now in the sea is more 

 than three times as great as the amount of calcium. 



The process of dolomitization may take place while the limestone is 

 still in the sea or after it has been raised above the sea and constitutes a 

 part of the land. 



Dolomitization before limestone emerges from the sea. Tlie Carbonate Originally depOS- 



ited by animals is almost wholly calcium carbonate. The sea contains 

 abundant magnesium salts. Thus we have in the sea a heterogeneous 



