20 BULLETIN 1311, U. S. DEPARTMENT OE AGRICULTURE 
is therefore important. We have such a comparison in the frac- 
tions extracted from the Huntington subsoil. The colloidal frac- 
tion between 0.3 and 1 micron, which is small in amount, differs 
from the composition of the total colloid below 0.3 micron, in being- 
higher in silica and lower in iron oxide and ignition loss. 
The different colloidal fractions from any one soil show a fair 
agreement, the more pronounced divergences generally occurring 
in the last fraction. 11 Although the last colloidal fraction may differ 
significantly in composition from the first, 12 the quantity of colloid 
in the last fraction is so small that it has little effect on the compo- 
sition of the entire amount of extractable colloid. It is apparent 
that the first fraction has almost exactly the same composition as 
all the colloid it was possible to extract. For example, the differ- 
ences in the silica contents of the first fraction and of the total 
colloid vary between 0.04 per cent for the Marshall and 1.33 per- 
cent for the Sharkey colloid. 
Since the first fraction is so closely representative of all the ex- 
tractable colloid, one would expect duplicate extractions to yield 
colloids of the same composition. This appears to be so. The com- 
positions of the Marshall and Huntington colloids given in Table 3 
are almost identical with the first fractions of these same colloids 
given in Table 5, although the samples of Tables 3 and 5 were ex- 
tracted at different times. 
CAUSES OF VARIATION IN COMPOSITION OF THE COLLOIDAL MATERIAL 
In considering the factors which may affect the composition of 
soil colloids it is necessary to trace briefly the process by which the 
colloids are formed. 
The original source of the inorganic material in soil colloids is 
the rocks of the lithosphere. When waters first appeared on the 
earth all the rocks must have been of igneous origin. 13 The min- 
erals of these rocks, being formed at high temperatures and in the 
absence of water, are not stable when bathed with water, as they are 
at most places on the surface of the earth ; consequently they break 
down, with the formation of a solution, which generally drains into 
the rivers, and a soil, which contains the colloidal residues. 
u Blanek and Preiss (6) found that the last colloidal fractions extracted from a certain 
soil were higher in silica and lower in alumina than the first fractions, and considered 
that the last fractions contained a greater proportion of finely divided quartz. It wiU 
be noted that the last fractions of the Huntington and Marshall colloids are also higher 
in silica and lower in alumina. This is not true of the Sharkey colloid and only partly 
true of the Vega Baja colloid. 
12 The composition of the colloids given in Table 3 is probably not appreciably affected 
by the process of extraction, as only 5 to 15 times as much water as soil was used. How- 
ever, in the extraction of successive fractions of colloids much more water was used, the 
ratio of water to soil varying between 200 and 2,000. Hence the last fractions of the 
successively extracted colloids were in contact with much more water than the first frac- 
tions of these colloids or the colloids whose analyses are given in Table 3. It will be 
shown that the water extracts of soils and colloids are comparatively high in lime. Soil 
colloids that have been treated with very large quantities of water would be expected 
to lose some lime through leaching. This may be the cause of the diminishing lime con- 
tent of the last fractions of the Sharkey and Vega Baja colloids, which were exposed to 
very large quantities of water. Much smaller quantities of water were used in the Hunt- 
ington and Marshall separations, and here the leaching effect of the water on the lime is 
not apparent. 
13 Clarke (S, p. 33, 116) estimates that the lithosphere contains 95 per cent of igneous 
rocks. The land surface is covered with 75 per cent sedimentary rocks and 25 per cent 
igneous rocks. The sedimentary rocks still contain large quantities of feldspar and per- 
haps other igneous minerals. In the coarser fractions of the soil, the minerals other than 
quartz are predominantly of igneous origin. 
