212 



CHKRT. 



The cherts of this arrangement must be ex- 

 pected to differ materially in aspect, when their 

 nature is considered ; as the rocks to which they 

 owe their origin, not only vary in composition, 

 but, may have undergone various degrees of in- 

 duration. In general, however, all the varieties 

 agree in being simple, in breaking difficultly, yet 

 suddenly, with a fracture more or lessconchoidal, 

 and, according to circumstances, either granular 

 or splintery, in forming acute cutting fragments, 

 in resisting steel, and refusing to effervesce with 

 acids. By this latter character they differ from 

 the limestones which they so often resemble, and 

 from which they are derived. The colours are 

 grey of various hues. 



GYPSUM. 



As a rock, this substance could only be con- 

 founded, by the student, with granular or primary 

 limestones ; and it is readily distinguished from 

 these by not effervescing with acids, by its com- 

 parative softness, and by the facility with which 

 it loses its water on exposure to heat. 



