996 A TREATISE ON METAMOEPHISM. 



nesium for calcium, or else abstraction of much calcium, or both. It is 

 plain that the magnesium which occurs in the sedimentary rocks as mag- 

 nesium carbonate is largely introduced by the substitution of magnesium 

 for previously precipitated calcium. 



When the sedimentary rocks and the altered original rocks of the zone 

 of katamorphism containing the secondary magnesium minerals pass into 

 the zone of anamorphism the original magnesium minerals, viz, the pyrox- 

 enes, amphiboles, micas, and olivines, may be produced; but a part of 

 the magnesium goes into other minerals. Of these the garnets appear 

 to be the most important, and some of the subordinate minerals are 

 chondrodite, humite, clinohumite, tourmaline, and melilite. 



It is a well-known fact that in mechanical sedimentary rocks meta- 

 morphosed in the zone of anamorphism the amphiboles are developed upon 

 a much greater scale than the pyroxenes, and that the magnesium-bearing 

 mica, biotite, forms very abundantly. This is a natural consequence of the 

 depletion of the mechanical sediments in calcium as compared with the 

 original rocks. The amphiboles are much more heavily magnesian than 

 the pyroxenes. For instance, in tremolite the magnesium-calcium ratio is 

 3:1, whereas in diopside it is 1:1. Consequently from the mechanical 

 products in which the magnesium is somewhat concentrated and the calcium 

 depleted there is a tendency to produce the magnesian minerals, amphi- 

 bole, and biotite. But in the development of amphiboles and pyroxenes 

 the pressure and the specific gravities of these minerals are also con- 

 cerned. (See pp. 278-280.) 



SODIUM. 



Sodium, according to Clarke's estimate of 1891, composes 2.28 per 

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

 including the original rocks, the hydrosphere, and the atmosphere. Accord- 

 ing to this estimate it composes 1.14 per cent of the hydrosphere and 2.36 

 per cent of the lithosphere. In his estimate of 1900 the amount of sodium 

 in the original rocks is increased to 2.63 per cent. Sodium thus has sixth 

 place among the elements. Reckoned as an oxide Clarke's estimate for 

 1891 is 3.20 per cent and for 1900 it is 3.55 per cent, and of oxides it 

 stands sixth. 11 



a Clarke, cit., Bull. 78, p. 39; Bull. 16S, p. 15. 



