284 CHERT. 



high interest in the estimation of the philosophical 

 geologist. If now first introduced into the system of 

 rocks, it could not have been omitted from an arrange- 

 ment including jasper and siliceous schist ; to which 

 it is intimately allied, by its transitions, and by its ge- 

 ological situation and origin. The reasons for adopt- 

 ing this form were given in the Classification of Rocks. 



As in the case of the siliceous schists, no general 

 definition of these cherts can be given. They often 

 resemble very accurately the mineral of that name 

 which is so nearly allied to chalcedony and flint. In 

 others, they may be compared to certain Species of 

 pottery, with which they correspond in the cleanness 

 and sharpness of their fracture, in the uniformity of 

 their texture, and in their hardness ; thus also, often 

 resembling porcelain jasper. They are most com- 

 monly simple ; but, occasionally contain imbedded 

 grains of quartz or felspar. Originating in limestone, 

 the transition from that rock into the chert, is often so 

 gradual, that no precise point can be assigned where 

 the term indurated limestone is no longer applicable. 

 It is repeating a preceding remark to observe, that 

 where trap occurs in contact with a complex series of 

 limestone, it will, in the same place, induce the crys- 

 tallization of the pure beds, and change the argillace- 

 ous ones into chert ; a now superfluous proof of the 

 source of this substance. 



Chert, like jasper and siliceous schist, occurs both 

 in the primary and secondary classes ; and for the same 

 obvious reasons, since the earthy limestones in which 

 it originates, are found in both. In the primary rocks, 

 granite, or porphyry, as well as recent trap, may pro- 

 duce that effect which, in the secondary class, can pro- 

 ceed from trap alone. To describe the geological 

 situations of this rock, would be therefore to repeat 



