GENERAL DEDUCTIOKS 225 



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

 bonic acid is never so small that the whole of the silica sep- 

 arated in the decomposition of feldspar can be removed.^ The 

 above, however, overlooks the possible presence of nitrates noted 

 on p. 154. The larger the proportion of nitric acid 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 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, as already noted. 



In cases where the decomposition takes place under the 

 influence of a sufficient supply of oxygen, all iron, and presum- 

 ably the manganese as well, would be converted into the in- 

 soluble hydrous sesquioxide form and remain with the residue. 

 Where, however, the supply of oxygen is insufficient, a por- 

 tion or all of these constituents may be removed in the form 

 of protoxide carbonates, or, in the case of iron, of a ferrous 

 sulphate. These facts well account for the variation in sta- 

 bility of the iron, as indicated in the preceding analyses. 



Eeference 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 cal- 

 careous stratified rocks. Eoth^ showed that in the weather- 

 ing of dolomitic limestones, the magnesia is often removed in 

 greater proportional quantities than the more soluble lime 

 carbonate. 



The researches of Hitterman^ showed, however, that carbonic 

 acid solutions may exert a scarcely appreciable effect upon mag- 

 nesian carbonate, which therefore accumulates in the residual 

 soils. 



It is safe to say that while the general process of rock- 

 weathering may be quite simple, as outlined, there are many 

 minor reactions which it is not possible to describe in detail. 



It has been shown that even in firm rocks a mutual chemical 



^Chemieal and Physical Geology, by Gustav Bisehof, Vol. II, pp. 183, 

 183. 



^Op. cit., YoL III. 



®Die Yerwitterungeproduete von Gesteinen der Triasformation Frankers, 

 Inang. Dissertation, Ereidxicli-Alexanders Universitat, Munich, 1889, 

 16 



