ANALYSES OF SHALES AND CLAYS. 893 



times as great as in the beach sand. Consequently, where the pressure is 

 sufficient to bring the particles, or a fair proportion of them, so close together 

 that they are within the limits of molecular attraction, welding is an 

 important factor in the coherence of the rock. It is well known that when 

 mud is subjected to very moderate pressure and the water can escape, this 

 is sufficient to make the particles cohere. There is therefore every reason 

 to believe that the weight of the superincumbent material accomplishes the 

 same thing for mud formations. 



Metasomatic processes are important in the induration of the muds. 

 The mechanical water within the interstices of the muds and that liberated 

 by dehydration are agents through which changes may be made within the 

 mineral particles and between the adjacent mineral particles. That dehy- 

 dration occurs is shown by comparison of the analyses of slates and 

 shales with those of muds. It has been seen on pages 742-744 that the 

 water of the slates and shales is, on the average, about 50 per cent of that 

 of the muds. 



Where rocks are so strongly hydrated as are the shales, some of the 

 water is combined with the hydrous minerals which most readily part with 

 it, such as the zeolites. Under such conditions the moderate pressures 

 and temperatures which produce the shales are sufficient forces to begin 

 dehydration. 



Where organic matter is present in muds — and this is very common — 

 the process of deoxidation is likely to begin. This is shown by the relative 

 proportions of the ferric and ferrous oxides in the muds and shales, as 

 exhibited in the analyses given on pages 890-891. It is seen that the ferric 

 oxide in the clays and soils averages 6.25 per cent, and in the shales is 

 reduced to 4.03 per cent; whereas the ferrous oxide in the clays and soils is 

 0.77 per cent and in the shales is increased to 2.46 per cent. These facts 

 accord with the statement in Chapter IV (p. 165; also see Chapter VII, 

 pp. 607-608) that deoxidation, while especially characteristic of the zone of 

 anamorphism, takes place in the belt of cementation where the conditions 

 are favorable. It thus appears that in the change from muds to shales we 

 have the beginning of reactions which for most rocks are not important 

 until they are deeply buried. Of the three reactions, hydration, oxidation, 

 and carbonation, which are characteristic of the zone of katamorphism for 

 the majority of rocks, two begin to be reversed for the pelites in the belt of 



