66 Dr. Mac Culloch on Jasper. 



It is to this last class of varieties that the term ought perhaps 

 to be limited ; as they cannot well be expressed by any other, 

 and as all the preceding substances may easily be ranked under 

 the several heads to which they strictly belong. Thus the term 

 jasper will become useful in a scientific view Avithout any material 

 innovation, merely by confining it to one of the best charac- 

 terized of the various substances to which it is indiscriminately 

 applied. 



If used in this sense it will be found that jasper occurs in 

 different geological situations, in many of which it is evidently 

 a substance changed, like the siliceous schists, from a different 

 original condition into its present form, in consequence of the 

 influence of trap, perhaps sometimes also, of granite. 



The most obvious case of this nature is where it is found in 

 beds, of greater or less extent, lying under masses of trap, or 

 else interstratified with it. In these cases its true origin is often 

 easily traced, as certain portions of the same beds will often be 

 found retaining their natural and original characters, apparently 

 from being more remote from the surrounding influence. The 

 analogy, in this case, between such jaspers and those artificial 

 substances known by the name of porcelain-jasper is very strik- 

 ing ; and it is scarcely necessary to point out their resemblance 

 in every respect to those that occur among volcanic rocks. The 

 transitions of this variety are generally into yellow clay, or into 

 the red iron clay which accompanies the trap rocks ; and the 

 colours accordingly vary. It is perhaps almost superfluous to 

 remark, that the same substances are occasionally found where 

 trap veins pass through strata capable of undergoing the same 

 change. This variety has occasionally been confounded with 

 pitchstone, as will immediately be explained. 



The transition into clay, here mentioned, points to the cause 

 to which the jasper in this case owes its origin, and forms an in- 

 teresting fact among many others in the history of the trap 

 rocks ; confirming the peculiar influence which they exert on all 

 those substances in contact with them, which are susceptible 

 of alterations from heat. The unchanged parts of the beds, in 

 these cases, are common ferruginous clay, red, yellow, or green, 



