262 Dr. Mac Culloch on a 



stances, can be conceived capable of producing the effects 

 under examination. Their partial and limited nature also 

 proves that two causes have acted in this case. 



The reader who has attended to the statement at the com- 

 mencement of this paper, is doubtless prepared to expect that 

 the prismatic structure in the instances which have now been 

 described are attributed to the action of heat. I shall attempt 

 to arrange the arguments which I have to offer in support of 

 this opinion in the most intelligible form. I make no scruple 

 in assuming that which all rational geologists now admit, 

 namely, that the trap-rocks are the produce of igneous fusion ; 

 and 5 in taking this for granted, the source of heat, so far as 

 it concerns these sandstones, is established. 



Now, in very many cases where trap rocks are found in 

 contact with certain strata, these are found to have undergone 

 particular changes within certain limited distances of the places 

 of contact. Thus, shale is converted into siliceous schist, 

 earthy limestone is crystallized, highly argillaceous limestone 

 becomes chert, and various argillaceous rocks pass into varieties 

 of jasper. 



The relations between the several varieties of this latter 

 substance thus produced, and the various natural or original 

 strata from which they have been formed, are those which 

 particularly bear on the present question. The purer clays 

 are found to be converted into fine jaspers of a resinous aspect, 

 as I have shewn in a former paper in this Journal. The 

 argillaceous white sandstones are changed into jaspers of a 

 less perfect character, which vary also in the same place, ac- 

 cording to previous varieties in the composition of the different 

 beds from which they have been derived, and to which they 

 may still be traced. The red argillaceous sandstones are con- 

 verted into red jaspers, of characters equally varying, accord- 

 ing to the previous nature of the original rock, or to its dis- 

 tance from the source of the heat ; and the examples of this 

 are so familiar to all practical geologists, as not to require 

 being specified. The shales which contain much siliceous 

 matter, are, in the same situations, converted into ironstone 

 and into jasper ; and, in the same manner, the incompact 

 highly ferruginous clays become, according to their capacity 



