INCIDENTAL COLOE CHANGES 243 



matter has been deposited either on the surface or between the 

 granules. 



(7) Changes in Color incidental to Weathering. — That in 

 nearly every rock a change in color, the assumption of a 

 brownish or reddish hue, is an early indication of decomposition 

 has been made sufficiently apparent in the chapter devoted to a 

 discussion of the chemical changes involved. This discolor- 

 ation is, however, merely incidental, and not essential, and is 

 found to diminish, if not wholly disappear, as the distance from 

 the surface increases, as was noted in the case of the granites of 

 the District of Columbia (p. 186) and the diorites of the Sierra 

 Nevadas. (P. 262. See further under Color of Soils, p. 373,) 



Granite and other highly feldspathic rocks carrying pro- 

 portionately small amounts of iron become almost invariably 

 bleached or w^hitened on the immediate surface, owing in part to 

 kaolinization and in part to the splitting up of the feldspars 

 along cleavage lines. 



In extreme cases rocks consisting of an admixture of feldspars 

 and iron-bearing silicates, but in which the feldspar, owing to 

 its glassy nature, is quite inconspicuous, become almost snow- 

 white .in the earlier stages of weathering. This as in the case 

 above mentioned, is due to the obscuring of the darker silicates 

 by the white product of kaolinization. Continued decomposition 

 must, however, attack the ferruginous constituent and the usual 

 staining ensue, unless, as in some eases possible, sufficient car- 

 bonic acid may exist to convert the iron immediately into car- 

 bonate and permit of its removal in solution. 



Allusion has been already made to the fact that oxidation 

 or other chemical action, with the possible exception of hydra- 

 tion, practically ceases below the permanent water level. Hunt 

 and Le Conte have both called attention to the fact that the 

 hornblendic and feldspathic rock fragments occurring in the 

 Pliocene auriferous gravels of California are firm and intact in 

 those portions below the drainage level (the blue gravel layer), 

 but more or less completely oxidized, kaolinized, and otherwise 

 altered in the red or upper gravel. 



Van den Broeck has called attention^ to the possibility that 

 the so-called red and gray diluvium of the Quaternary deposits 

 near Paris may be but portions of one and the same geological 

 body, the ^^ diluvium rouge^' being but an upper member of 



^Bull. Soc. Geologique de. France, 5, 1876-77, p. 298. 



