REDISTRIBUTION OF SULPHUR. 959 



reduced to sulphide by organic matter and other reducing agents, and in 

 this form is precipitated. These precipitates are mainly those of the heavy 

 metals, especially of iron. A considerable portion of the sulphates which 

 form in the belt of weathering and are carried to the belt of cementation 

 is brought to the surface by solutions which join the overground circulation. 

 By this circulation sulphates are transported to the sea. It has already 

 been noted that a large amount of sulphur is found in the sea as sulphate. 

 The amount now present in solution is not necessarily any measure of the 

 amount which has been contributed to the ocean. Indeed the relatively 

 large amount of sulphates in the shales suggests that where immense masses 

 of mud have become mingled with the sea water, as at the mouths of great 

 rivers, reactions take place which result in the precipitation of the sulphates, 

 and these salts thereby become mingled with the muds which later are 

 transformed to shales. While by far the larger portion of the sulphates 

 of the overground circulation are carried to the sea, in areas such as the 

 Great Basin calcium sulphate is thrown down in the lakes and gypsum 

 deposits are built up. Such deposits are of very considerable magnitude. 

 So far as these gypsum deposits formed in the zone of katarnorphism pass 

 into the zone of anamorphism they may become dehydrated and anhydrite 

 be formed. 



Gypsum and barite are of very considerable economic importance in 

 various ways. In these products a very large amount of sulphur is concen- 

 trated. The segregated sulphur, which, combined with the other elements, 

 makes these products of value, has been brought together in the condensed 

 form from the fraction of 1 per cent of the sulphur in the original rocks. 

 Thus we have another illustration of the manner in which processes of 

 metamorphism segregate elements, and result in the formation of deposits 

 which are of service to man. 



SILICON. 



Silicon stands second in abundance among the elements of the earth. 

 According' to Clarke's estimate of 1891 silicon forms 25.30 per cent of the 

 lithosphere, hydrosphere, and atmosphere together, and of the lithosphere 

 alone 27.21 per cent, a little more than one-fourth of the total. According 

 to Clarke's estimate of 1900 silicon composes 28.06 per cent of the litho- 

 sphere. 11 All of this silicon is combined. In analyses silicon is usuallv 



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



