550 JASPER. 



tions, of no determinate form or character, in 

 those parts only of the beds, where the granite or 

 trap to which its origin is referred, are immedi- 

 ately present. 



Lastly, it exists in the form of veins, often 

 very minute ; and, in these cases, it is probably 

 a mere modification of some venous rock of the 

 trap family, analogous to that case where basalt 

 becomes, in the progress of ramification, con- 

 verted into pitchstone. 



Jasper presents a few modifications of internal 

 structure which require notice. It sometimes 

 gives indications of a spheroidal concretionary 

 disposition, more or less perfect, and resembling 

 that which, under circumstances of a similar 

 nature, occurs in chert and siliceous schist In 

 the same way, it sometimes possesses a laminar 

 structure, arid thus also it approximates to the 

 siliceous schists. It is easy to see bow, from 

 similarity of origin, connexions, and composition, 

 it may be thus a matter of doubt to which of those 

 two rocks, any given specimen or bed should be 

 referred. The well-known striped and spotted 

 jaspers, owe their appearance to the two struc- 



