162 iiuAV CROPS FEEb, 



pure quartz. The freestone of tlie Connecticut Valley la 

 a granitic sandstone, containing fragments of feldspar 

 and spangles of mica. 



Other varieties are calcareous, argillaceous {clayey)^ 

 basaltic, etc., etc. 



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 

 argillite. 



Limestones of the sedimentary kind are soft, compact, 

 nearly lusterless rocks of various colors, usually gray, 

 blue, or black. They are sometimes nearly pure carbon- 

 ate of lime, but usually contain other substances, and are 

 often higlily impure. When containing much carbonate 

 of magnesia they are termed raagnesian limestones. They 

 pass into sandstones through intermediate calciferous 

 sand rocks, and into sliales through argillaceovs lime- 

 stones. These impure limestones furnish the hydraulic 

 cements of commerce. 



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. 



