992 A TREATISE ON METAMORPHISM. 



stones are formed. While much of the CaC0 3 produced by carbonation is 

 transported to the sea, a considerable part of it is transported to the belt of 

 cementation, and enough of this is precipitated to make CaC0 3 a cementing 

 agent second in importance to silica. (See pp. 624-625.) A very subor- 

 dinate j3art of the CaC0 3 in the belt of cementation enters into combination 

 with silica, forming the zeolites and epidotes. 



From the foregoing it appears that the calcium in the secondary 

 minerals of the zone of katamorphijsm is largely in the form of carbonate, 

 often not a simple carbonate, but a calcium-magnesium carbonate. In this 

 zone also are found other secondary calcium-bearing minerals, such as the 

 zeolites and epidotes. 



When the rocks containing these secondary calcium-bearing minerals 

 pass into the zone of anamorphism, the minerals are decomposed, the carbo- 

 nates b}^ silication and the zeolites and epidotes by dehydration. In the 

 nearly pure calcium-magnesium carbonate rocks the silicates which form 

 most plentifully are wollastonite, diopside, tremolite, and actinolite, Natur- 

 ally, where the limestones are nearly pure calcium carbonate, by silication 

 the pure calcium silicate, wollastonite, forms. If magnesium is not very 

 abundant, there is likely to be developed the pyroxene, diojjside, in which 

 the calcium-magnesium ratio is 1:1. Where the carbonate rocks are 

 stronger magnesian, tremolite forms; and where they also bear iron, 

 actinolite develops. In the rocks in which there is a considerable number 

 of bases, such as the impure limestones, the shales, and the sandstones, 

 other silicates form. Any of the calcium-bearing silicates occurring in 

 the original rocks may be reproduced, and part of the calcium usually, 

 passes into heavy minerals not commonly found in the original rocks, 

 such as garnets, melilite, the scapolites, etc. 



MAGNESIUM. 



According to Clarke's estimace of 1891, magnesium forms 2.50 per cent 

 of the outer 10 miles (16.1 kilometers) of the crust of the earth, including 

 the original rocks, the ocean, and the atmosphere. Of the ocean it 

 comprises 0.14 per cent, and of the original rocks 2.68 per cent. In his 

 estimate of 1900, Clarke reduces the amount for the original rocks to 2.62 

 per cent." Magnesium thus stands seventh in the scale of abundance. It 

 is about five-sevenths as abundant as calcium, the element with which it is 



a Clarke, cit., Bull. 78, p. 39; Bull. 168, p. 15. 



