JASPER. 277 



The forms of jasper necessarily vary according to 

 the original characters of the parent substance. It 

 sometimes occurs in obscure irregular masses, like 

 limestone or serpentine ; while, at others, it appears 

 to consist of true strata. The foregoing remarks ex- 

 plain this latter form ; while the irregular one must 

 occur when it constitutes an incidental member of the 

 overlying rocks. It is equally obvious how it may oc- 

 cur, either among the primary or secondary rocks, in 

 small portions, of no peculiar form or characters 

 these being regulated by the actual contact or in- 

 fluence of trap or granite at those places. It is also 

 found in the shape of veins, sometimes very minute. 

 In some cases, these seem analogous in their origin to 

 trap veins ; as when basalt, in the progress of ramifi- 

 cation, becomes pitchstone : in others they appear to 

 have been portions of clay entangled among trap, and 

 indurated instead of being fused. 



Jasper presents some modifications of structure re- 

 quiring notice. Such are the indications of a sphe- 

 roidal concretionary one similar to that which occurs 

 in siliceous schist ; and thus also it sometimes pos- 

 sesses, like these, a schistose structure. The common 

 striped and spotted jaspers proceed from these modi- 

 fications ; and it is thus easy to understand how it 

 may often be difficult to distinguish between the sili- 

 ceous schists and the jaspers. A minute columnar 

 structure is sometimes also found, resembling that of the 

 argillaceous ironstones, or ferruginous shales. This 

 is readily understood ; because these very shales are 

 converted into jasper by the contact of trap, and the 

 limits between the two are often undefinable. This 

 columnar structure is occasionally peculiar, being 

 divided by protuberant joints, and striped parallel to 

 the axes, so as to resemble madreporite limestone. 



