REDISTRIBUTION OF MAGNESIUM. 995 



below, being much more largely retained in the belt of weathering and in 

 the belt of cementation than is calcium. But as yet no data are available 

 to enable us to make any statement as to the quantitative importance of 

 this concentration. 



To what extent the magnesium in the ocean, in the saline deposits, and 

 segregated in the zone of katamorphism will explain the deficiency in the 

 ordinary sedimentary rocks can not be stated. This can be determined only 

 after a careful quantitative estimate has been made of these various supplies 

 of magnesium. The redistribution of magnesium by metamorphism, while 

 in many respects analogous to that of calcium, is in many respects also differ- 

 ent. It has been seen that by the process of carbonation of the silicates 

 by far the larger part of the calcium is released from silica and changed to 

 carbonates. The major part of the calcium carbonate and other carbonates, 

 such as sodium carbonate, find their way to the sea in solution. While the 

 process of carbonation also changes a considerable portion of the magnesium 

 of the silicates to magnesium carbonate, a very large part remains in the 

 silicates. The olivines change to serpentine. The nonaluminous pyroxenes 

 and amphiboles alter to talc on an extensive scale. The aluminous pyrox- 

 enes and amphiboles pass into chlorite, a magnesium aluminous silicate. 

 The pyroxenes extensively change to amphibole, and this involves a partial 

 substitution of magnesium for calcium, or else a subtraction of calcium and 

 partial carbonation, or both. Thus, the magnesium is concentrated in the 

 zone of katamorphism, and especially in the belt of weathering. But this 

 concentration is largely due to the abstraction of other elements rather than 

 the addition of magnesia. 



The magnesium of the original silicates, which is changed to carbonate 

 in the zone of katamorphism, goes through the same processes of distribu- 

 tion as the calcium carbonate. A part of it passes down into the belt of 

 cementation, and is there precipitated in dolomite, ankerite, parankerite, 

 and hydrous magnesium silicates. But a large part of the magnesium salts, 

 before or after an underground journey, join the surface streams and find 

 their way to the ocean. Magnesium is there precipitated on a great scale 

 as carbonate, not by the reaction of organisms, but by the reaction of 

 magnesium salts in solution upon the calcium carbonate previously precipi- 

 tated by organisms. This process inaugurated in the sea is continued in 

 the carbonate formations on the land. This involves a substitution of mag- 



