256 A TREATISE ON METAMORPHISM. 



in the soil horizon. Wherever the feldspathic rocks are exposed to atmos- 

 pheric agencies this change steadily goes on, though not so rapidly as with 

 the orthosilicate feldspars. (See p. 519.) But wherever the potash feld- 

 spars have been very long exposed to the weathering agencies they have 

 been partly or wholly decomposed, and in some places to a depth of several 

 hundred feet. The change is one of the most important of all those which 

 affect rocks. It is partly because the alterations take place near the 

 surface, where carbon dioxide is abundant, that it is believed that the freed 

 alkali largely unites with carbon dioxide, as given in the reaction. The 

 silica freed in the belt of weathering is in part undoubtedly taken into 

 solution as colloidal silicic acid and carried downward to the belt of 

 cementation. Indeed, the silica for the process of silicification in this belt, 

 which, as explained on page 480, is derived from the decomposition of the 

 silicates, probably in good part comes from the alteration of the feldspars. 

 Under the same conditions in which a part of the feldspar breaks up into 

 kaolinite another part of the feldspar may produce gibbsite, quartz, and 

 potassium carbonate. The potassium carbonate liberated at the time of 

 the formation of the kaolinite and gibbsite is largely dissolved and trans- 

 ported elsewhere, although the soluble potassium compounds are often held 

 in the soil to a considerable extent. (See pp. 498, 541-543.) 



The alteration of orthoclase and microcline to minerals of the kaolin 

 group and to gibbsite is not, however, confined to the belt of weathering. 

 It takes place on an important scale in the belt of cementation, though not 

 on a scale comparable to that in the belt above. So far as known, kaolin- 

 ization is not a reaction which occurs in the zone of anamorphism; at least, 

 if it does there take place it is a very subordinate phenomenon. As seen 

 above, the reaction is one taking place with liberation of heat and fre- 

 quently with decrease of volume, since much and perhaps the most of the 

 freed silica is taken away in solution. The heat reaction controls, and 

 hence the change is under the rules of the upper physical-chemical zone. 



The alteration of orthoclase and microcline to mica occurs in rocks 

 which have been somewhat deeply buried, and the change has been noted 

 in connection with both mass-static and mass-mechanical action. Under 

 either of these conditions the alteration may be nearly or quite complete. 

 But it has taken place on the most extensive scale in connection with mass- 

 mechanical action, where the secondary structures, such as cleavage, are 



