CLIMATE AND TERRESTRIAL DEPOSITS 293 



oxidation of the ferruginous material in the flood-plain deposits is 

 accomplished. This supplements the decomposition at the source 

 and that which takes place in the long transportation and great wear 

 to which the larger rivers subject the detritus rolled along their beds. 

 The annual wetting, drying, and oxidation not only decompose the 

 original iron minerals but completely remove all traces of carbon. 

 If this conclusion be correct, red shales or sandstones, as distinct from 

 red mud and sand, may originate under intermittently rainy, subarid, 

 or arid climates without any close relation to temperature and typically 

 as fluvial and pluvial deposits upon the land, though to a limited extent 

 as fluviatUe sediments coming to rest upon the bottom of the shallow 

 sea. The origin of such sediment is most favored by climates which 

 are hot and alternately wet and dry as opposed to climates which are 

 either constantly cool or constantly wet or constantly dry. 



The origin of light and variegated colors. — White or light gray in 

 rocks indicates an absence of iron, an absence which may be due to 

 entirely mechanical causes, as in white clean sandstones of either 

 fluvial, pluvial, marine, or aeolian origin; or to chemical causes, as in 

 gray or black clays. 



Aeolian deposits whether coarse or fine, as dune sand or loess, are 

 typically light in color, varying from nearly white to pale yellow or 

 pink. The lightness in color appears to be due in dune sand to the 

 mechanical segregation of the grains, but in loess partly to the lack of 

 decomposition, partly to the lack of diffusion of the coloring substance. 

 The preceding discussion upon the significance of red would suggest 

 that loess deposits older than the Pleistocene should be dominantly 

 colored from white to light pink rather than the more customary 

 pale buff of the recent deposits. Upon weathering, however, such a 

 buff color would tend to be to some extent restored on account of the 

 porous nature and the new decomposition which is possible in the 

 case of loess. 



Where lightness in color is due to chemical causes it is to be noted 

 that the first result of the action of fermenting organic matter upon 

 ferruginous clays is a change of color from rusty to bluish or greenish 

 by the reduction of ferric to ferroso-ferric hydrate. Afterward, if 

 the action be continued, the solution of ferrous carbonate may be 

 formed, and the greenish or bluish color may disappear. The impor- 



