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mixed with the kaolin formed, the greater the quantity of carbonic 

 acid in water, and that, perhaps, the amount of carbonic acid in 

 water is never so small that the whole of the silica separated in 

 the decomposition of feldspar can be removed.^ The above, how- 

 ever, wholly overlooks the possible presence of nitrates, such as 

 we now know from the researches noted on p. 857 must in many 

 cases exist even though in extremely small proportions. It is 

 probable that the small amounts of nitric acid formed by the bac- 

 teria would, if not at once takenupby plants, combine immediately 

 with the alkalies, potash, or soda, forming nitrates which, owing 

 to their ready solubility, would be carried away. The larger the 

 proportion of nitric acid, therefore, the greater would be the 

 amount of carbonic left, and consecjuently the greater would be 

 the amount of silica intermingled with the kaolin, since whatever 

 proportion of the alkalies failed to be carried away as nitrates 

 would pretty certainly disappear as a carbonate. There is also 

 the possibility, especially in the rocks rich in iron protoxides, 

 that a portion of the silica may combine with the iron. (See 

 Bischof, Vol. II, p. 'j^.) 



In cases where the decomposition takes place under the influ- 

 ence of a suf^cient supply of oxygen, all iron and presumably 

 the manganese as well would be converted into the insoluble 

 hydrous sesquioxide form and remain with the residue. Where, 

 however, the supply of oxygen is insufficient, a portion or all of 

 these constituents may be removed in the form of protoxide car- 

 bonates, or, in the case of iron, as a sulphate. 



Reference has already been made to the fact that the mag- 

 nesia from the decomposition of magnesian silicates was some- 

 times removed in greater relative portions than was the lime. 

 This seeming anomaly is also sometimes met with in calcareous 

 stratified rocks. 



Roth^ states that in the weathering of dolomitic limestones 

 the magnesia is often removed in greater proportional quantities 

 than the more soluble lime carbonate. 



'Chemical and Physical Geology, Gustav Bischof, Vol II, pp. 182, 183. 

 = Op. cit., Vol. III. 



