
SOILS AND SUBSOILS 375 
come to form beds of pure quartz-sand and fine clay, 
composed of hydrous silicate of alumina alone, with, in 
most cases, some proportion of the finest quartz-flour—all 
soluble ingredients having been removed. 
The Soil-cap.—Ifthe soil-cap, therefore, consists essentially 
of disintegrated rock-materials, it is obvious that it must 
vary very much in character. In some places it will contain 
a high percentage of soluble matter—in other places it will 
contain little or none at all—and between these extremes 
all gradations may be expected to occur. The character 
of the soil-cap being thus dependent upon that of the 
underlying rocks, a good geological map might be expected 
to throw much light on the present distribution of soils, 
And so, indeed, it does; nevertheless, other factors have had 
their influence upon the distribution of soils, and unless these 
are kept in view, a geological map may be misleading. The 
colours upon such maps have reference, as a rule, only 
to the so-called “solid rocks,’ and these may or may not 
crop out at the actual] surface. Asa matter of fact, they are 
often deeply buried underneath superficial accumulations of 
cravel, sand, clay, loam, peat, etc. Wide regions may be 
represented on the map, therefore, as being occupied by 
limestone, or by sandstones and shales, or other strata— 
although none of these may actually appear at the surface— 
the only exposures being those seen in stream- and river- 
courses. All the intervening low grounds may be thickly 
mantled with superficial deposits. In cases of this kind, 
therefore, the soils take their character from the overlying 
deposits, and not from the covered and concealed bed-rock. 
It is hardly necessary to say that under such conditions the 
soil-cap may differ very considerably in character from that 
which the solid rocks would have yielded. Again, the actual 
configuration of the ground must influence the distribution 
of soil. All loose disintegrated rock material tends to travel 
downwards. Rain, alternate frost and thaw, etc., slowly or 
more rapidly, as the case may be, cause soil-ingredients to 
pass from higher to lower levels—thus the disintegrated 
materials derived from one kind of rock may invade and 
overlie the outcrops of, it may be, totally different strata. 
The character of a soil may even be very considerably 
