SEDENTARY SOILS 31 



11. Sedentary soils. Sedentary soils are of two 

 kinds: (1) Residual, or soils consisting of the residue 

 left behind in rock decomposition. (2) Cumulose, or 

 soils resulting from the slow accumulation and decay of 

 plant remains. 



12. Residual soils. There may be as many kinds 

 of residual soil as there are rocks. Because of similarity 

 between the species in a group of rocks, a few of these 

 groups may be considered as types. The most promi- 

 nent groups are the igneous rocks, the calcareous rocks, 

 shale or slate and sandstone. Attention will be directed 

 as far as possible to the relation of the soil composition 

 to the composition of the original rock and to the char- 

 acter of the material lost in the transition. 



In calculating the relative loss of the different ele- 

 ments in the transition process, some one element 

 usually iron or aluminum, and, in the case of limestone, 

 silicon is assumed to have suffered no loss. This 

 method, adopted from Merrill, is, of course, not strictly 

 accurate, since every element is subject to losses; but 

 it serves as a fair comparative basis for the study of the 

 loss of plant-food elements. 



The important areas of residual soil in North America 

 occur south of the limit of glaciation, which extends 

 roughly from New York to Cincinnati, thence to St. 

 Louis and up the Missouri river to the Dakotas, and 

 west to the Sound region of Washington, where it again 

 loops well to the south. The residual soils are further 

 hemmed in by coastal deposits, which have their greatest 

 extent in the South Atlantic and Gulf Coast region, 

 where they reach a width of more than a hundred miles. 



