OVERLYING AND TRAP ROCKS. 103 



trap. I have pursued miles of a porphyry lying on 

 argillaceous schist, believing it more antient than the 

 secondary strata : yet a single vein intersecting the red 

 sandstone or the coal series, has in a moment reduced 

 this date. Undoubtedly, there are porphyries more 

 antient, ranking with the granites, perhaps of more 

 ages than one. But whatever the apparent proofs 

 may be, we can do nothing unless we could command 

 the negative facts also ; and this cannot be. The 

 porphyry of Inverary and that of Kirkcudbright ap- 

 pear of different ages ; but there is a secondary rock 

 within reach to betray the date of the latter, while 

 there is not one to tell us what the former might have 

 been. 



I would indeed willingly separate the antient " por- 

 phyries" from the modern ones and the traps, because 

 of their respective connexions and dates, if I knew how 

 to do it ; since it would be convenient, until geologists 

 are willing to receive what I dare not now attempt, a 

 proposal to treat of granite, " porphyry," and trap as 

 a single family, being convinced that geological philo- 

 sophy must adopt this, sooner or later. But if, in se- 

 parating granite from trap, I have had no other 

 resource than a conventional criterion, founded on the 

 assumption that granites must not " overlie" the strata, 

 and that traps may, there is not even this expedient 

 for separating the antient " porphyries" from the mo- 

 dern ones and the other traps of later date. All of 

 them " overlie" equally, all overlie all strata, all pro- 

 duce similar veins with similar effects, all have been de- 

 posited under fluidity of fusion on the rocks then 

 uppermost, and all have suffered waste, like the strata 

 themselves ; so that even a conventional distinction is 

 impossible. And if we seek that in rnineralogical 

 characters, these cannot be trusted. There are an- 



