106 HOW CROPS FEED. 
which is remarkable for its extraordinary and persistent 
fertility. The prairies of our own West, the bottom lands 
of the Scioto and other rivers of Ohio, are other examples 
of peculiar soils; while on every farm, almost, may bs 
found numerous gradations from clay to sand, from vege- 
table mould to gravel—gradations in color, consistence, 
composition, and productiveness. 
CHAPTER IL 
ORIGIN AND FORMATION OF SOILS. 
Some consideration of the origin of soils is adapted to 
assist in understanding the reasons of their fertility. 
Geological studies give us reasons to believe that what is 
now soil was once, in chief part, solid rock. We find in 
nearly all soils fragments of rock, recognizable as such by 
the eye, and by help of the microscope it is often easy to 
perceive that those portions of the soil which are impal- 
vable to the feel are only minuter grains of the same rock. 
Rocks are aggregates or mixtures of certain minerals. 
Minerals, again, are chemical compounds of various ele- 
ments. 
We have therefore to consider: 
I. The Chemical Elements of Rocks. 
II. The Mineralogical Elements of Rocks. 
IIL The Rocks themselves—their Kinds and Special 
Characters. 
IV. The Conversion of Rocks into Soils; to which we 
may add: 
VY. The Incorporation of Organic Matter with Soils. 
