THE PHYSICAL PROCESSES OF SOIL FORMATION. n 



by weathering; second, clay, as one of the chief results of the 

 weathering /process of silicate minerals; and thirdly, humus, the 

 darK-colored remnant of vegetable decay. According to the 

 obvious predominance of one or the other of these primary 

 ingredients, soils are popularly, in the most general sense, 

 classed as "heavy" and "light"; the former term corre- 

 sponding as a rule to those in which clay forms a prominent 

 ingredient, while sandy and humous or " mold " soils usually 

 fall under the latter designation, because of their easy tillage. 

 For practical purposes these subdivisions are both convenient 

 and important, and they form the ordinary basis of land class- 

 ification. Beyond these, the degree of fineness of the rock de- 

 bris, and their physical and chemical constitution, determine 

 distinctions such as gravelly, sandy, silty, loamy, calcareous, 

 siliceous, magnesian, ferruginous, and others of less general 

 application, though locally often of considerable importance. 



For the purposes of discussion and definition, however, an- 

 other basis of classification is needed, which essentially con- 

 ns both the origin and the adaptations of lands. 



UPLAND 

 Plateau 



'Scab , o,il Sedentary Soil 



Land 



FiG. 5. Diagram illustrating the genetic relation of different soil classes to each other. 



I. Sedentary Soils. When soils have been formed without 

 removal from the site of the original rock, by simple weather- 

 ing, they are designated as sedentary, or residual soils, or 

 " soils in place." In the case of these, the original rock under- 

 lies the soil or subsoil at a greater or less depth, according to 

 the intensity and duration of the weathering process, and is 

 usually more or less softened and decomposed at the surface 

 where it meets the soil layer. The latter of course bears some 

 of the distinctive characters of the parent rock, and its composi- 

 tion and adaptations may. in a measure, be directly inferred 

 from that origin. Such soils usually contain, especially in their 

 lower portions, some angular fragments of the parent rock. In 



