CHERT. 565 



was stated in the two preceding chapters ; and it 

 will abbreviate the account of its history, to say, 

 that it bears the same relations to the argillo- 

 siliceous limestones, or the calcareous sandstones, 

 that the siliceous schists do to slate and shale. 



It occurs therefore among the primary strata ; 

 yet rarely, as the primary limestones are generally 

 so pure as only to undergo a change to the crys- 

 talline texture, by those causes which convert the 

 argillaceous varieties into chert. For the same 

 reasons, nearly, it forms, among the primary 

 rocks, very limited masses ; occurring chiefly, 

 if not only, in those cases where granite or por- 

 phyry are in contact with limestones of the cha- 

 racter just mentioned. 



Among the secondary strata, it is often found 

 in extensive portions; passing into the common 

 limestone by intermediate stages of induration, 

 or forming entire strata, varying in character 

 according to the circumstances under which it is 

 associated with the accompanying trap rocks, or 

 according to the previous composition of the 

 original limestone. In these cases, it is generally 

 associated with siliceous schist, as the unchanged 



