452 SECONDARY LIMESTONE. 



a. Imperfectly granular; shining. 



b. Earthy. 



c. Smooth : the fracture splintery or ap- 



proaching to the conchoidal. 



This variety becomes so overcharged with the 

 other earths, as to pass into calcareous shale, or 

 marie slate. 



E. Carbonat of lime, more or less pure, and in- 

 termixed with bitumen. Bituminous limestone. 



F. Carbonat of lime intermixed with a large 

 proportion of silica chiefly. Effervesces with great 

 difficulty, unless in powder : farms slags in the fire : 

 passes into calcareous sandstone. 



When near trap, it is often highly indurated, 

 even previously to its assuming the character of 

 chert, and it then frequently presents a very ano- 

 malous appearance. These varieties effervesce 

 with still greater reluctance, unless reduced to 

 powder. 



THIRD DIVISION. 



Compound : containing some visible ingre- 

 dient intermixed. 



A. Limestone containing mica. 



B. Limestone containing mica and sand. 



These varieties occur in company with mica- 

 ceous, or micaceo-arenaceous shale. 



