960 A TREATISE ON METAMORPHISM. 



reported with its combined oxygen as silica. Stated as silica, Clarke's 

 estimates of 1891 and 1900 are 58.59 and 59.71 per cent, respectively. 01 

 This is the average, and in the original rocks the silica rarely falls below 50 

 per cent and rarely rises above 75. Of the elements present in meteorites 

 it has been seen that silicon is third in abundance. Much the larger portion 

 of the silicon in the original rocks occurs as silicates, although a large 

 amount occurs as silica in quartz. 



The amount of silica in 78 shales is 58.38 per cent, in 624 sandstones 

 is 81.76 per cent, and in 843 limestones is 9.64 per cent. (See p. 938). 



Tt is plain from the foregoing that within the lithosphere metamorphism, 

 denudation, and sedimentation have redistributed the silica to a very 

 important extent. In the shales the amount remains about the same as 

 the average for the original rocks. In the limestones the amount of silica 

 has been very greatly reduced; indeed, to less than one-sixth of that in the 

 original rocks. Complementary to this depletion of silica in the limestones 

 is its segregation in the sandstones. In these rocks the silica is a little 

 more than one-third greater than the average for the original rocks. This 

 small difference is due to the fact that the silica in the sandstones is mainly 

 in the form of quartz ; while in the original rocks, as already noted, the 

 larger part is in the silicates, of which it composes about two-thirds. 

 Therefore, while the percentage of silica in the lithosphere as a whole is 

 two-thirds as great as in the sandstones, the amount of silica as quartz in 

 the lithosphere, 12 per cent (see p. 937), is only about one-sixth as great 

 as in the sandstones, for the majority of sandstones contain 75 per cent or 

 more of silica. 



Considering the zone of katamorphism, there is constant solution of 

 silica in the belt of weathering and steady additions of silica in the belt of 

 cementation, already fully exjolained. (See pp. 473-480, 516-517, 621- 

 623, 634-636.) Carbonation in the belt of weathering* destroys the silicates 

 and liberates silica as colloidal silicic acid. This is largely carried down- 

 ward into the belt of cementation and is there deposited in enormous quan- 

 tities. (Seep. 618.) Hence we have constant subtraction of silica from the 

 belt of weathering and its steady addition to the belt of cementation. In 

 the belt of weathering the proportion of silica may be increased or decreased 

 according to the rate of solution of the other elements. (See pp. 507 et seq.). 



'Clarke, cit,, Bull. 168, p. 14. 



