JASPER. 551 



tures above mentioned ; and, occasionally, the 

 two are combined in the same specimen. 



It is much more rare to find jasper possessing 

 a minute columnar structure resembling that of 

 the madreporite limestone, or of ironstone. But 

 this, when it occurs, is easily explained, when it 

 is recollected that it often differs from this latter 

 substance, only in the degree of hardness. The 

 transition into ironstone, is similar to that into 

 the ferruginous clay of the strata which lie under 

 trap. One instance of the columnar structure on 

 the large scale occurs at Dunbar* 



The texture, and of course, the fracture, of 

 jasper, differs in the several varieties. The frac- 

 ture is either flat, or imperfectly conchoidal, or 

 very accurately so ; and, in the minute fractures, 

 it is imperfectly granular, or angular and rough, 

 or splintery, or dull and pulverulent, or uniformly 

 smooth. The last variety, where it emulates 

 pitchstone, or common pitch, in lustre, has often v 

 a fracture as perfectly conchoidal as that sub- 

 stance. 



From these characters, and from the defini- 

 tion given in the thirteenth chapter, it is pre- 



