JASPER. 275 



used in this sense, it will be found to occur in different 

 geological situations ; being evidently changed, like 

 the siliceous schists, from a different original condi- 

 tion to its present form, through the influence of trap, 

 and sometimes also, of granite. 



The most obvious case of this nature, is where it 

 occupies beds, of various extent, lying under masses 

 of trap, or interstratified with it. Here, its true origin 

 is often easily traced ; as certain portions of the same 

 beds will often be found to retain their natural and 

 original characters, from having been less exposed to 

 the surrounding influence. The analogy between such 

 jasper and those semi-artificial substances termed por- 

 celain-jasper is very striking ; and I need scarcely 

 point out its resemblance in origin to those which 

 occur among volcanic rocks. The transitions of this 

 variety are into the coloured clays which attend the 

 trap rocks, or else into the argillaceous sandstones ; 

 while the qualities and the colours vary accordingly : 

 these having formed ihe original beds, and this result 

 being an important fact in the history of trap also. 

 Thus, a green clay, in Iceland, produces green jasper: 

 in Sky, yellow and red ones, of a resinous lustre, ac- 

 company beds of yellow and red clay ; and in Salis- 

 bury craig, and at Stirling castle, where red and yel- 

 low argillaceous sandstones have undergone the same 

 change, the accompanying jaspers correspond to them 

 in colour and in mineral character. In the latter, the 

 resinous lustre is either wanting or obscure ; and thus 

 it is easy to account for the various texture and aspect 

 of this multifarious family. Those which have a re- 

 sinous lustre with a conchoidal fracture and a smooth 

 surface, have been confounded with pitchstones, even 

 by mineralogists of reputation ; giving rise to the in- 

 correct notion that pitchstone was a stratified rock ; 



T 2 



