CLIMATIC AND GEOCHEMICAL RELATIONSHIPS 77 



or some intermediate product. The composition of the 

 common iron oxides found in soils tends to support 

 Crosby's explanation : 



Hematite .... Fe 2 O 3 Red 



Turgite .... 2 Fe 2 O 3 . H 2 O 



Goethite .... Fe 2 O 3 . H 2 O 



Limonite .... 2 F 2 O 3 . 3 H 2 O 



Xanthosiderite . . Fe 2 O 3 . 2 H 2 O 



Limnite .... Fe 2 O 3 . 3 H 2 O Yellow 



Merrill l holds the same idea, but thinks that the sur- 

 face soil may contain relatively more iron than the sub- 

 soil. He considers that the ferric iron oxides, because 

 of their insoluble nature, tend to accumulate at the 

 surface, and because of their large quantities and because 

 they are there subjected to more vigorous weathering 

 action the vivid red colors tend to develop. 



The iron coloring matter usually exists as a coating 2 

 on the soil particles, although it may sometimes occur 

 as concretions. It is found also that in general, but not 

 always, the intensity of the color varies with the amount 

 of iron present. From a large number of analyses com- 

 piled by Robinson and McCaughey, 3 the following figures 

 may be obtained showing the authority for such a state- 

 ment : 



AVERAGE IRON CONTENT OF PERCENT OF 



SOILS FERRIC IRON 



Deep reds to light reds 14.40 



Ochre yellow to yellow 8.85 



1 Merrill, G. P. Rocks, Rock Weathering, and Soils, p. 375. 

 New York. 1906. 



2 Van Bemmelen, J. M. Beitrage zur Kenntnis der Ver- 

 witterungsprodukte der Silicate in Ton-, Vulkanischen-, und Lat- 

 erite-Boden. Zeit. Anorg. Chem., Bd. 42, Seite. 290-298. 1904. 



3 Robinson, W. O., and McCaughey, W. J. The Color of 

 Soils. U. S. D. A., Bur. Soils, Bui. 79, p. 21. 1911. 



