CHERT. 569 



F. With a smooth glistening fracture, angular 

 and sharp ; extremely sonorous, brittle, and translu- 

 cent on the edges. 



G. With the aspect of chalcedonic quartz, and 

 passing into it. 



Some examples of this variety have been 

 called stratified quartz. It originates in calca- 

 reous sandstones. 



SECOND DIVISION. 



Compound. 



A. Containing interspersed crystals of quartz, 

 or, occasionally, of glassy felspar, and thus somewhat 

 porphyritic. 



The colours of this substance are various. It 

 is most commonly pale, or smoke grey, more 

 rarely dark grey, sometimes of a dull white. 

 It is occasionally reddish brown, or purple brown, 

 or nearly black, or mott;ed of several colours. 



It passes at one extreme into limestone, or 

 into quartz, according to its origin, and occasion- 

 ally presents an appearance of crystalline plate 

 interspersed. 



