REDISTRIBUTION OF SODIUM. 999 



degree -with potassium. During- the underground journey of a part of the 

 solutions a small portion of this sodium is deposited in the belt of cementa- 

 tion. The sodium-bearing minerals formed hy direct alteration in the zone 

 of katamorphism, and those there deposited as cementing minerals, are 

 substantially the same and are mainly the zeolites. The more important 

 of these zeolites containing- sodium in notable quantities are thomsonite, 

 hydronephelite, natrolite, mesolite, analcite, stilbite, and gmelinite. 



So far as rocks containing sodium in the zeolites pass into the zone of 

 anamorphism some of the original minerals of which sodium is a constituent 

 may be reproduced, but manifestly the amount of such minerals will be 

 small as compared with the amount of the original sodium-bearing minerals 

 in the rocks. Corresponding with this fact the rich sodium minerals, 

 nephelite, cancrinite, sodalite, haiiynite, and noselite, have never been 

 reported as anamorphic minerals of the sedimentarv rocks. Most of the 

 sodium present passes into the feldspars; but where the rocks have been so 

 far decomposed as not to leave a large quantity of the original sodium- 

 bearing minerals, the amount of sodium is not sufficient to produce any 

 considerable proportion of the feldspars. In consequence, as pointed out 

 on pages 899-904, the schists and gneisses which form from pelites only 

 exceptionally contain any considerable quantity of soda feldspars. The 

 dominant secondary minerals which are produced are quartz and mica. 

 However, in the psephites, in which the processes of decomposition are not 

 far advanced, and in which the sodium is therefore relatively abundant, 

 considerable quantities of soda feldspars are produced during recrystalli- 

 zation. But as already noted, even in those sedimentary rocks in which 

 residual soda is present in the undecomposed orig-inal minerals, it appears 

 that it is not known to have been anywhere so abundant that the peculiar 

 minerals known as the soda-bearing minerals, such as nephelite, sodalite, 

 etc., are produced. 



POTASSIUM. 



According- to Clarke's estimate of 1891 potassium composes 2.23 per 

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

 including the original rocks, hydrosphere, and atmosphere. Of the ocean 

 it composes 0.04 per cent, and of the original rocks 2.40 per cent. In his 

 estimate of 1900 Clarke reduces the figure for the original rocks to 2.32 



