OVERLYING AND TRAP ROCKS. 121 



often equally impossible to determine whether there 

 is any difference in the aeras of the several deposits of 

 trap, or whether they may not, according to this hy- 

 pothetical statement, have heen all parts of one gene- 

 ral formation. I have, however found the means of 

 proving that such deposits have taken place at different 

 and distant times, and that some of them are of an 

 origin prior to the formation of the secondary strata. 



In the lowest red sandstone in different parts of 

 Scotland, there occur imbedded masses of these sub- 

 stances, in considerable quantity and variety, and 

 bearing the marks of attrition. These materials must 

 have been derived from primary rocks ; and thus the 

 existence of trap formations prior to the deposition of 

 the secondary strata is established. And lest it might 

 be inferred that these were merely the varieties of 

 granite which I have described, I must remark that 

 many of them are amygdaloidal, and some cavernous; 

 as they all resemble the later members of this family. 

 There are circumstances in the veins of these sub- 

 stances traversing the primary rocks, which lead to 

 the same inference of antiquity as to them, if they do 

 not absolutely prove it, since they rarely if ever occur 

 in those which traverse the secondary strata. Their 

 base is that compact felspar, generally coloured by 

 the peroxide of iron, which distinguishes the well- 

 known porphyries, while the crystals of felspar are 

 rarely absent, and commonly abundant, compared to 

 their occurrence in the later veins. They also fre- 

 quently contain mica and quartz, which are rare in the 

 later, together with pinite, which has never yet been 

 found in these ; presenting other characters less easily 

 defined than recognised, while frequently also assum- 

 ing a granitic character. And if such veins never are 

 traced from the primary rocks into the secondary. 



