KERRERA. GEOLOGY. 121 



of any original connexion with the primary previously 

 to the deposition of those, it might be concluded that 

 it was universally posterior to the secondary rocks ; and 

 that as granite is the unstratified rock which peculiarly 

 interferes with the primary strata, so trap was the intrud- 

 ing substance in the secondary. From the present fact 

 it is however evident, that there have been masses of 

 trap, similar to the most recent in every circumstance 

 of structure and composition, but of a date prior to 

 that of the secondary rocks; and it offers perhaps the 

 only unquestionable species of evidence on that subject 

 which can be obtained. It will still always remain 

 uncertain, unless where similar circumstances can be 

 proved, whether the trap rocks now actually found in 

 conjunction with the primary strata, are of primary or 

 secondary date ; as alternation affords no evidence of 

 consecutive formation, in the case of rocks which possess 

 the intruding character. Other characters from whence 

 such evidence may be derived, may possibly hereafter 

 be discovered; and the subject is thus pointed out to 

 geologists as one of the numerous important circum- 

 stances still required in the general history, not only of 

 this particular family, but of the science itself. 



In the present case it became a natural object of inquiry 

 whether the trap of Appin, which is conterminous with the 

 granite of Loch Etive, might not have been the source 

 of the nodules in question. The examination of this 

 tract however, proved, that it was associated with beds 

 of conglomerate containing fragments similar to those 

 of the sandstone of Oban and of the island under review ; 

 and that it was therefore in every respect to be consi- 

 dered as a portion of the general trap district of Mull 

 and Lorn. 



In concluding I may remark, that the rarity of trap 

 rocks among those of the primary class, presuming that 

 they have actually existed in this period as in the 

 secondary, may be explained from the great facility of 



