PRINCIPLES OF ROCK WEATHERING J I J 



fill solvent and othcM' chemical reactions which have been instru- 

 mental in promoting rock destruction, and it is here, then, that 

 we will consider the complex chemical processes commonly 

 grouped under the head of oxidation, deoxidation, hydration, 

 and solution. 



Oxidation. — -Oxidation is .perceptibly manifested only in 

 rocks carrying iron either as sulphide, protoxide carbonate, or 

 silicate. The sulphides, in presence of water and when not 

 fullv protected from atmospheric influences, readily succumb, 

 producing sulphates, which being soluble are rapidly removed 

 in solution, or hydrated oxides, sulphuretted hydrogen, and 

 perhaps free sulphur. Such an oxidation is attended by an 

 increase in bulk so that if nothing escapes by solution there 

 may be brought to bear a physical agency to aid in disintegra- 

 tion. Weathered rocks containing iron sulphides may not infre- 

 quently be found with cubical cavities quite empty or partially 

 filled with the brownish, yellow, or red product of their oxidation, 

 in a more or less powdery condition. Pyrites, though a wide- 

 spread constituent, is nevertheless a less conspicuous agent in 

 promoting rock decomposition than the protoxide carbonates 

 and silicates. In these the iron passes also over to the hydrated 

 sesquioxide state, as is indicated by the general discoloration 

 whereby the rock becomes first streaked and stained and finally 

 uniformly ochreous. The more common minerals thus attacked 

 are the ferruginous carbonates of lime and magnesia and sili- 

 cates of the mica, amphibole, and pyroxene groups. As the 

 oxidation progresses the minerals become gradually decom- 

 posed and fall away into unrecognizable forms. The red and 

 yellow colors of soils are due invariably to the iron oxides con- 

 tained bv them. 



Deoxidation is a less common feature than oxidation. Water 

 carrying quantities of organic acids may, through their influ- 

 ence, take awa}- a portion of the combined oxygen of a sesqui- 

 oxide, converting it once more into the protoxide state, in 

 which form it may be dissolved and removed as a ferrous car- 

 bonate or sulphate. The local bleaching of certain ferruginous 



