122 HOW CROPS FEED. 
pure quartz. The freestone of the Connecticut Valley is 
a granitic sandstone, containing fragments of feldspar 
and spangles of mica. 
Other varieties are calcareous, argillaceous (clayey), 
basaltic, etc., ete. 
Shales are soft, slaty rocks of various colors, gray, green, ! 
red, blue, and black. They consist of compacted clay. 
When crystallized by metamorphic action, they constitute 
arg illite. 
Limestones of the sedimentary kind are soft, compact, 
nearly lusterless rocks of various colors, usually gray, 
plue, or black. They are sometimes nearly pure carbon- 
ate of lime, but usually contain other substances, and are 
often highly impure. When containing much carbonate 
of magnesia they are termed magnesian limestones. They 
pass into sandstones through intermediate calciferous 
sand rocks, and into shales through argillaceous lime- 
stones. These impure limestones furnish the hydraulic 
cements of commerce. 
g§ 4. 
CONVERSION OF ROCKS INTO SOILS. 
Soils are broken and decomposed rocks. 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 impalpa- 
ble to the feel chiefly consist of minuter grains of the same 
rock. 
Geology makes probable that the globe was once in a 
melted condition, and came to its present state through a 
process of cooling. By loss of heat its exterior surface 
solidified to a crust of solid rock, totally incapable of sup- 
porting the life of agricultural plants, being impenetrable 
to their roots, and destitute of all the other external char- 
acteristics of a soil. 
