68 Dr. Mac Cullotli on Jasper. 



among the secondary claystones where these are in contact with 

 masses or veins of trap. The cases are here analogous to those 

 first mentioned ; but such local jaspers are often found to possess 

 a peculiar concretionary structure ; being either laminar, or 

 formed of spheroidal or other analogous concretions. These 

 are the substances frequently found forming the spotted and 

 striped jaspers of collectors. 



Lastly, jasper occurs in irregular masses among the primary 

 rocks, occupying situations analogous to porphyry, claystone, 

 or trap, but presenting no transitions into these by which to 

 indicate a similarity of origin. That origin appears neverthe- 

 less to be, in some cases, the same : but, on this part of the 

 subject, we are yet in want of much information. 



From the preceding statement, it is apparent that jasper must 

 belong indifferently both to the primary and to the secondary 

 division ; and it would be an unnecessary sacrifice to an imperfect 

 arrangemeiit, in this as in many other cases, to form two divi- 

 sions of this rock, or to separate it into primary and secondary 

 from any considerations of this nature. 



The forms of jasper vary according to these several circum- 

 stances of position. Like limestone or serpentine, it is some- 

 times found in irregular masses, obscurely, or not at all stratified. 

 In other cases, in the primary rocks, it appears to form true 

 strata ; a circumstance to be expected. Among the secondary 

 rocks, it is massive and shapeless where it passes into claystone, 

 and is stratified where it forms a portion of the series of strata 

 connected with trap. 



As it is also found in a state of transition into the ordinary 

 stratified rocks, in both classes, it is easy to conceive how it 

 •may occur in small portions, 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 immediately 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 be- 

 comes, in the progress of ramification, converted into pitchstone. 

 Jasper presents a few modifications of internal structure which 



