REDISTRIBUTION OF SILICON. 961 



By erosion the belt of cementation, enriched in silica, rises into the belt of 

 weathering and the rocks rich in quartz are broken down. The larger 

 part of the quartz is not chemically modified, but mechanically disintegrated, 

 contributed to the streams, and by them carried to the sea. It there joins 

 the quartz liberated from the rocks of the shore by the action of the waves, 

 and the whole is segregated in sandstone formations. Many of the sand- 

 stone formations thus produced in early geological time later became land 

 areas. During later geological time streams running over sandstone areas 

 contributed to the sea more than an average amount of quartz. Thus the 

 combined processes of metamorphism and denudation have resulted in 

 segregating sufficient quantities of quartzose sands to produce the quartz- 

 sand formations and their metamorphosed equivalents, the quartzites and 

 quartz-schists. 



In the deep-seated zone of anamorphism silica is neither added nor 

 subtracted in any considerable quantity, but the form of the compound is 

 often extensively changed. As fully explained (pp. 677-679), silica unites 

 with the bases, especially those of the carbonates, to produce silicates. 

 Whether at the present time the destruction of the silicates in the zone of 

 katamorphism, and especially in the belt of weathering, or the development 

 of the silicates and the destruction of the carbonates in the zone of 

 anamorphism is more important is considered under the heading "Carbon." 

 (See pp. 962-974.) 



The proportion of silica in the different classes of sediments gives a 

 criterion by which the estimates of the relative amounts of the sediments 

 (see pp. 940-941) may be very roughly tested. If the shales, sandstones, 

 and limestones compose 0.65, 0.30, and 0.05 of the sediments, respectively, 

 and contain the percentages of silica above given, the sum of the multiples 

 of the percentage of silica in each of the sediments by the quantities of 

 the sediments should equal the average percentage of silica in the original 

 rocks. Thus 



58.38X.65+81.76X.30+9.64X.05=62.957. 



But this equation shows an excess of 3.247 per cent of silica over that 

 determined by analysis, 59.71 per cent. 



Practically all of the silicon occurs in the lithosphere. If all classes of 

 sedimentary rocks were taken into account, including both their composition 

 and mass, an estimate of the percentage of silica based upon the sediments 

 MON xlvii — 04 61 



