THE COLOB OE SOILS 373 



In many cases this regur is derived directly from basaltic 

 rocks, by surface decomposition in situ, whilst other varieties 

 were derived from other aluminous rocks, or are alluvial deposits 

 in river valleys, lakes, lagoons, and marshes. The dark color, as 

 is usual, is due to the presence of organic matter.^ 



The term subsoil is applied to that portion of the regolith 

 which immediately underlies the soil proper, from which it 

 differs mainly in compactness, and the smaller amount of oxi- 

 dation and decomposition it has undergone. In a soil which 

 has never been cultivated, the sub-soil may pass gradually up- 

 ward into the soil without distinct lines of demarcation. Pro- 

 longed cultivation may, however, have so thoroughly oxidized 

 and physically altered the superficial portions down to the limit 

 of plough and root action, as to bring about a very marked differ- 

 ence, both in color and texture, as well as in actual composition. 

 At times the sub-soil becomes so thoroughly compacted as to be 

 almost impervious, forming a so-called hardpan, 



(6) The Color of Soils. — The color of soils is due mainly to 

 carbonaceous matter and iron oxides. To the first are due the 

 dark gray to black colors characteristic of prairie and swamp 

 soils. To iron oxides are due the buff, yellow, ochreous-brown, 

 and red hues, the source of the oxides being mainly the silicate 

 minerals from whence the soils were derived. It sometimes 

 happens, as abundantly demonstrated in the southern Appa- 

 lachian states, that it is possible in passing over any section 

 of the country to designate with a fair degree of accuracy 

 the lithological nature of the underlying rocks from the color 

 of the residual soils, even though the rocks themselves may 

 be wholly obscured by decomposition products. In such cases 

 rocks rich in iron silicates, like hornblende and augite, give 

 rise to bright red soils, while those poor in these constituents 

 yield soils of a gray or slightly yellowish hue. Much, however, 

 depends on extent of decomposition and on climatic conditions, 

 as noted below. 



One of the most striking features of the landscape observed 

 in traveling southward along the Appalachian belt is the abrupt 

 transition in color of the soil, as the limit of glacial action is 

 past. "Within the glaciated area, except where derived directly 

 from highly colored rocks, like the Triassic sandstones, the soils 

 are everywhere dull in color, some shade of gray, drab, or brown. 



^ Manual of the Geology of India, 2d ed., by B. D. Oldtam, 1893, p. 411. 



