CHERT. 567 



milar circumstances in all the associated sub- 

 stances. 



The texture, and the corresponding fracture of 

 this rock, vary in different specimens, and chiefly 

 according to the perfection of its character, or to 

 the degree in which it differs from the limestones 

 and sandstones from which it has originated , The 

 texture is, therefore, more or less compact and 

 uniform, and the fracture either dull and earthy, 

 or smooth. The form of the fracture is always 

 plain, or conchoidal, on a large or small scale ; 

 and, in those specimens which are highly com- 

 pacted, the conchoidal fracture is generally very 

 perfect, and the lustre considerable, but resem- 

 bling that of the most compact limestones. It is 

 further a remarkable part of its character, that, 

 although generally yielding with the greatest dif- 

 ficulty to the hammer in the larger masses, the 

 smaller fragments often fly asunder with great 

 violence, and present cutting edges as sharp as 

 glass. To compare its fragments to those of pot- 

 tery and porcelain, is to convey the most accurate 

 idea that can be given of this part of the charac- 

 ter of these particular varieties. 



