GEOLOGICAL CLASSIFICATION OF SOILS 33 



Arkansas Limestone and its Residual Clay l 



Si0 2 



A1 2 3 



Fe 2 0.- 



MnO 



CaQL 



MgO 



K 2 



Na 2 



C0 2 



Percentage 

 Lost 



.00 

 11.35 



. 89.56 

 57.59 

 98.93 

 89.38 

 66.36 

 53.26 



100.00 



The vast age of such soils tends to bring about great 

 oxidation, so that most of the iron has changed to hematite 

 and limonite. Since almost all soils contain considerable 

 iron, the prevailing colors of residual soils are reds and 

 yellows, depending on the degree of oxidation and hydra- 

 tion. Grays and browns may exist, however, where iron 

 has been lacking or oxidation has been feeble. In texture 

 such soils usually present very fine conditions. Having 

 been attacked by both the physical and the chemical 

 agencies, the particles have been reduced to a very fine 

 state of division. Over residual areas the heavier soils 

 predominate, as silts, silt loams, clays, and clay loams. 

 Very often sand or chert may l>e present, having been a 

 constituent of the original rock mass. 



An examination of the particles of a residual soil usually 

 shows them to be in an advanced stage of decay. The 

 feldspars have lost their luster and have become opaque. 

 The iron has become oxidized, and the soluble bases have 



1 Penrose, R. A. F. 

 Vol. I, p. 179. 1890. 



D 



Ann. Rept. Geol. Survey Arkansas, 



