CAlsK OK SOILS in<:i) coioi; 10.") 



shown in tlie iVactional sei)arations made by washing these residual 

 chiys and sands, whereb}' they are separated into proportional parts of 

 var\*ing degrees of tineness.'' It has been recently shown in climates 

 where rock weathering is accompanied by a leaching process, in which 

 the more soluble constituents are more or less gradually removed in 

 solution, the ferric and alumina salts are found to be the most refrac- 

 tory of the essential constituents in igneous rcxjks. The resulting final 

 l)roduct of decomposition consists therefore of a highly ferruginous clay, 

 composed mainl}' of the hydrated aluminous silicates, free iron sesqui- 

 oxides. and, in the case of acid rocks, additional free silica. 



Merrill* remarks that, all things being equal, the dej)tli or degree of 

 color indicates advanced decom[)osition and also geological age. This 

 statement accords well with the writer's work on the Georgia granites. 

 The gray soils derived from the granitoid rocks of the state represent not 

 an advanced stage of decomposition, but mainly a process of disintegra- 

 tion manifested in the partial kaolinization of the feldspars and a slight 

 rusting, oxidation of the biotite ; while the red soils derived from the same 

 rocks are invariably deep, red ferruginous clays in which the biotite is 

 thoroughly leached and oxidized and the feldspars nearly, if not entirely, 

 kaolinized. Stated somewhat differentl}^ the gray soils contain an abun- 

 dance of all the component minerals present in the fresh rock, but slightly 

 altered chemically ; are highly porous, sandy soils, from which clay is 

 essentially absent, and have been derived from the fresh rock chiefly by 

 disintegration. The red soils, on the other hand, contain comparatively 

 only a very small percentage of the undecomposed silicate minerals; 

 are deep red, ferruginous, stiff clays, rendered more or less gritty from 

 the presence of free quartz, and have resulted from decomposition. The 

 gray soils represent, therefore, the earlier stages, disintegration, in the 

 weathering of the rocks, while the red soils are conversely the products 

 of decomi)osition and represent the advanced stages in the change. 



While quantity of pigment — ferric oxide — seemingly controls or rather 

 accounts for the brilliant coloring of the soils in general, conditions 

 afford strong indications, at least, of the suggestion of presence of some 

 other factor or factors of marked importance. In support of this state- 

 ment the author has critically examined all available authentic analyses 

 of the decayed rock products and carefully noted each writer's descrip- 

 tion as to color. One or two illustrations will suffice : 



Dr Merrill f has described the weathering of a micaceous gneiss from 

 Albemarle county, Virginia, the decayed product — residual soil — of 



* Rocks, Rock Weathering, and Soils, 1897, p, ri80. 

 Bull. Geol. Soc. Am., 1897, vol. 8, pp. 157-168. 



