EXTENSION COURSE IN SOILS. 9 
Which are the two most common rock-forming minerals of the earth's crust? 
(b) Examine with the lens the different rock samples — granite, trap rock, schist, 
shale, slate, limestone, marble, sandstone, and quartzite. Compare these with the 
mineral samples and try to determine from which minerals the rocks were largely- 
formed. What kind or kinds of soil are formed from granite? From sandstone? 
From limestone? From shale and slate? 
PHYSICAL AND CHEMICAL CHANGES. 
Examine a piece of rock candy, noting color, crystalline form, hardness, and taste. 
Grind a piece to powder with the mortar and pestle. Has the taste changed? Dis- 
solve a little of the powder in a small quantity of water. Taste the liquid to determine 
if the material still exists. The changes thus far have been physical changes. Now 
heat a little of the powder in a dish, slowly first, noting all the changes. Heat until 
no further changes take place, then allow to cool. Taste the residue. Note its color. 
What does the new substance resemble? Will it dissolve in water? What kind of a 
change has taken place? 
SOIL COMPOSITION. 
Material composing soils. — Examine carefully (hand lens may be used) several 
samples of soil in the field or classroom and note their physical make-up. Distinguish 
between organic and inorganic particles, between vegetable and mineral matter. 
Which contain more vegetable matter, the light or dark colored soils? Are the 
organic and inorganic particles distinct and separate, or do they adhere closely to one 
another? What is the source of the vegetable matter? The mineral matter? 
Mineral base of soils. — Examine carefully these samples again and note the variation 
in size of the mineral particles. What name is given to the large mineral particles? 
Of what may these particles consist? What name is given to the fine, dustlike parti- 
cles? Of what may these particles consist? What are the intermediate-sized grains 
called? Do you find particles of these sizes in greater or less abundance in all samples 
examined? 
Mineral particles determined by sedimentation. — Place a tablespoonful of soil in a long 
pickle bottle and fill the bottle up to the neck with water; add a few drops of ammonia; 
shake well for at least three minutes. Set down the bottle and observe the settling 
of soil particles. The material which settles to the bottom during the first few seconds 
is coarse sand or gravel. The material which continues to settle more slowly during 
the next few minutes is silt. The water is turbid after settling has apparently ceased 
because of the fine clay particles in suspension. Put aside the bottle and find how 
long some of the fine particles will stay in suspension. 
FIELD STUDY. 
Where practicable, field trips or excursions may be made for studying the rock 
formation of the community, noting relations between the prevailing rocks and the 
types of soil. It should also be noted whether the particular areas of soil visited 
are of residual or transported formation. 
REVIEW QUESTIONS, LESSON I. 
1. Of what two parts does soil largely consist? From what does each part originate? 
2. Name some common rock-forming minerals. What are sedimentary rocks? 
3. What is humus? Explain how it is produced in the soil. 
4. What is a physical change? A chemical change? A biological change? Give 
examples of each. 
5. What is an element? A compound? Give examples of each. 
