KINDS OF SOILS. 151 
‘What is designated as clayey substance, or impalpable 
matter, is oftentimes largely made up of rock-dust, so fine 
that it is supported by 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, already described, p. 113, under the 
mineralogical name of kaolinite, 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 minerals. So gradual is the transition 
from true kaolinite clay through its impurer sorts to mere 
impalpable rock-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- 
eal 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 originated 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 humus. 
As Schone has shown, (Bulletin dela Societé des Natura- 
listes de Moscou, 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 
