GEOLOGICAL CLASSIFICATION OF SOILS 39 



the rocks tliat underlie them, if sound, often given some clue 

 to the character and composition of the parent material.^ 

 Under such conditions the changes that a rock undergoes in 

 forming a soil may be studied to the best advantage. 



Residual soils are usually non-stratified and present a 

 heterogeneous mass of material, grading from a true soil, 

 with its normal content of organic matter, downward through 

 the typical soil material to the unweathered country rock 

 below. Since such soil has been subject to leaching for long 

 periods, a very large amount of its soluble materials have 

 been washed out, tending to leave high percentages of the 

 persistent elements, such as silica, iron and aluminum. The 

 preceding discussion of soil formation has already emphasized 

 this phase sufficiently. 



The great age of residual soils has given opportunity for 

 very thorough oxidation, so that much of the iron has changed 

 to hematite or to the hydrated limonite group. The yellow 

 color of the latter group is indicative of greater age than the 

 former. Since almost all soil material contains considerable 

 iron the prevailing colors of residual soils are reds and yel- 

 lows, depending on the degree of oxidation and hydration. 

 Grays, browns, and blacks often occur, however, where oxida- 

 tion has not progressed or where organic matter is present in 

 amounts suiScient to mask the iron coloration. 



As residual soils have been subjected to intense physical 

 and chemical weathering, the particles have been reduced 

 to a very fine state of division. Over residual areas the 

 heavier types, such as silt loams, clay loams and clays pre- 

 dominate. Sands and sandy loams may occur, however, 

 when the parent rock carried considerable quartz and a low 

 percentage of clay-producing minerals, such as feldspar, horn- 

 blende and augite. Soils from limestones, granites, and 



^Eesidual soils are not always derived from rock similar to that 

 directly underlying the soil as is often assumed. "When the present 

 bed rock is much different from the stratum which gave rise to the 

 soil, the soil is said to be ' ' inherited. ' ' 



