566 CHERT. 



calcareous strata are with shale. Where it is the 

 result, merely of the passage, of trap veins, it is 

 limited to a short space from the contact of these, 

 and passes gradually into the ordinary strata 

 whence it is derived. 



In those instances where chert originates in 

 the change of beds of calcareous or calcareo- 

 argillaceous sandstone, it sometimes forms exten- 

 sive masses, or involves a considerable series of 

 strata, varying in character, and often approach- 

 ing in aspect to quartz : and, in some cases, as 

 may easily be understood, where the original 

 limestone is highly argillaceous, it approximates, 

 either to jasper, or to siliceous schist, according 

 to circumstances ; so that it might sometimes be 

 as properly referred to either of these. It will 

 illustrate the present views of its origin, to remark, 

 'that where a vein or mass of trap is in contact 

 with different strata of limestone varying in cha- 

 racter, the purer varieties become crystalline, 

 while the impure are converted into chert. 



The only variety of concretionary structure it 

 has hitherto presented, is the spheroidal ; and it 

 is a remarkable fact, that this occurs under si- 



