622 PRINCIPLES OF STRATIGRAPHY 



Mississippic beds of Nova Scotia, where the !ron has "the aspect 

 of a chemical precipitate rather than of a substance triturated me- 

 chanically" (2o:i'5). The same thing has been noted by Hilgard 

 with regard to the red tropical soils, where the percentage of 

 ferric oxide is by no means markedly high, but where the oxide 

 is very finely diffused throughout the mass. 



Finely diffused oxide of iron, but in the state of ferric hydrate, 

 is the coloring agent of the yellow loess of China. The total amount 

 of ferric oxide in American loess ranges from 2.50 per cent to 3.74 

 per cent., and in one case to 5.22 per cent., while a limited amount 

 of FeO (from 0.12 to 1.02%) is also present, but organic carbon 

 is very slight, ranging from 0.09 to 0.19 per cent. Dehydration of 

 the iron with age, especially after burial, would result in the forma- 

 tion of fine-grained red deposits, in every respect similar to tlie 

 Vernon red shales. 



The red Vernon shales, like red shales and sandstones fre- 

 quently, are associated with salt and gypsum deposits. This is 

 an indication of arid conditions during the deposition of the muds 

 from which these shales are formed. Salt and gypsum deposits of 

 the present time are associated with gray and bluish-gray muds 

 and it is only by subsequent oxidation and dehydration that the 

 red color is produced. 



One of the essentials in the production of red rocks by such 

 subsequent oxidation and dehydration appears to be the virtual 

 absence of organic matter which would prevent oxidation. Where 

 the ground water level is high organic matter will accumulate and 

 oxidation is prevented. But where the sands and muds are ex- 

 posed to circulating air in dry seasons, with a low ground water 

 level, Tnore or less complete oxidation of the iron will take place. 

 Such a condition is found in delta deposits of arid regions, as, for 

 example, the Nile delta, in which organic matter seems to be of 

 very small amount, while the iron content is comparatively high. 

 It is probable that the Nile mud on aging would take on a red color. 



There is, however, another factor which may affect the change 

 in color, and that is the lime carbonate content of the deposit. This 

 is high in many delta deposits of arid regions and its presence may 

 prevent the production of a red color by the formation of complex 

 silicates of lime, iron, and alumina. According to Ries (47 '.6, 11) 

 a buff or cream color is produced if clay containing three times as 

 much lime as iron or more is burned into bricks. Magnesia has 

 the same coloring effect on the burned ware as lime, while alkalies 

 tend to turn the iron red into a brown. As the Rhine delta in the 

 Lake of Constance has nearly 12 times as much lime carbonate as 



