548 JASPER. 



tion into clinkstone or compact felspar, become 

 true jasper, as far as the external characters can 

 be relied on; a change which need excite no 

 surprise, when their various composition is con- 

 sidered. Thus jasper is, rigidly speaking, an 

 occasional member of the trap family ; but, on 

 account of its prevailing characters and positions, 

 it was judged proper to treat of the whole in this 

 place. It is only one instance, out of many, of 

 the impossibility of framing any arrangement 

 which shall be in all points unobjectionable. 



In many situations where the trap rocks cover 

 or interfere with the secondary strata, jasper is 

 found in contact with the former, and connected 

 with the latter by an imperceptible transition. 

 This transition points to the cause to which such 

 jasper owes its origin ; and it will be found that 

 the strata indurated into this new form, are beds 

 of clay, more or less ferruginous, or of argillace- 

 ous sandstone. Thus the jasper presents transi- 

 tions into ordinary yellow clay, into the red ferru- 

 ginous clay so common among the trap rocks, arid 

 into sandstone. In Iceland a green variety is thus 

 found passing into a clay highly coloured by 

 chlorite. 



