12 SOILS. 



some cases sedentary soils may have been partially derived 

 from rocks that have been removed from above the present 

 country rock by erosion, and in that case fragments of such 

 vanished rock may also be present. 



Sedentary soils are most commonly found on rock plateaus 

 and on slopes or plains underlaid by rock strata of but slight 

 inclination, where the velocity of the " run-off " rainfall is not 

 sufficient to dislodge the rock debris. Extended areas of such 

 soils exist in the granitic areas of the southern Alleghanies, in 

 the " black prairies " of the Cotton States, and on the " basal- 

 tic " plateaus of the Pacific Northwest. 



2. Colliic'ial Soils. \Yhen the soil mass formed by weath- 

 ering has been removed from the original site to such a degree 

 as to cause it to intermingle with the materials of other rocks 

 or layers, as is usually the case on hillsides, and in undulating 

 uplands generally, as the result of rolling or sliding down, 

 washing of rains, sweeping of wind, etc., the mixed soil, which 

 will usually be found to contain angular fragments of various 

 rocks, and is destitute of any definite structure, is designated 

 as a collni'ial 1 one. Colluvial soil ma e- are frequently sub- 

 ject to disturbance from landslides, which are usually the result 

 of water penetrating underneath, bet \\een the soil mass and 

 the underlying rock, <>r sometimes simply of complete satur- 

 ation of the former with water. Aside from such catastrophic 

 action, they commonly have a slow downward movement in 

 mass (creep), which ordinarily becomes perceptible only in 

 the course of years; nio>t quickly \\here there are heavy frosts 

 in winter, which act both by direct expansion, and by the state 

 of extreme looseness in which the soil mass is left on thawing. 

 Colluvial soils form a large portion of rolling and hilly up- 

 lands, and are of very varying degrees of productiveness, 



3. Allurial Soils. \Yhen soils are the result of deposition 

 by streams, the material having been gathered along the course 



1 The term " overplaced," UM'd for such soils in late memoirs of tin- I*. S. r,eo- 

 logk-.'il Survey, is at least superfluous, in view of the perfectly understood term 

 already in general use, and does not seem to commend itself f<>r adoption l>y any 

 special or superior fitness; nor does the suggestion of Shaler (Tin- Origin and 

 Nature of Soils, I2th Rep. I". S. Cicol. Survey) to include the colluvial soils within 

 the alluvial class, commend itself either from a theoretical or practical point of 

 of view, since but few useful generalizations can apply to both classes. 



