KINDS OP sorrA 151 



What is designated as clayey substance^ or impalpable 

 matter^ is oftentimes largely made up of rock-dust, so fine 

 that it is supported hy water, when the latter is in the 

 gentlest motion. In what are properly termed clay-soils, 

 the finest parts consist, however, chiefly of the hydrous 

 silicate of alumina, aln-ady described, p. 113, under the 

 mineralogical name of haol'mlte^ or of analogous com- 

 pounds, mixed with gelatinous silica, oxides of iron, and 

 carbonate of lime, as well as with finely divided quartz 

 and other granitic miner.ils. 80 gradual is the transition 

 from true kaolinite clay through its impurer sorts to mere 

 impalpable lock-dust, in all that relates to sensible char- 

 acters, as color, feel, adhesiveness, and plasticity, that the 

 term clay is employed rather loosely in agriculture, being 

 not infrequently given to soils that contain very little 

 kaolinite or true clay, and thus implies the general physi- 

 cal qualities that are usually typified by clay rather than 

 the presence of any definite chemical compound, like 

 kaolinite, in the soil. 



Many soils contain much carbonate of lime in an im- 

 palpable form, this substance having been derived from 

 lime rocks, as marble and chalk, from the shells of mollusks, 

 or from coral ; or from clays that have oiiginated by the 

 chemical decomposition of feldspathic rocks containing 

 much lime. 



Organic matter, especially the debris of former vegeta- 

 tion, is almost never absent from the impalpable portion 

 of the soil, existing there in some of the various forms as- 

 sumed by hunnis. 



As Schone has shown, {Bulletin de la Societe des JVcttura- 

 listes de Moscoa, 1867, p. 363), the results obtained by 

 Nobel's apparatus are far from answering the purposes of 

 science. The separation is not carried far enough, and no 

 simple relations subsist between the separated portions, as 

 regards the dimensions of their particles. If the soil were 

 composed of spherical particles of one kind of matter, or 



