78 GENERAL COMPARISON OF 



rock of which I have not met with any corresponding 

 instance. 



There is so wide an interval between the trap of Morven 

 and the nearest analogous rocks which occur on the 

 mainland, that it at first appears scarcely justifiable to 

 suppose any connexion between them. I allude to the 

 great tract of claystone and porphyry, which extends 

 interruptedly from Ben Nevis through Loch Leven to 

 the head of Loch Etive, and includes Glenco. But if 

 the interval between this tract and Mull is considerable 

 in this direction, it is materially contracted when we trace 

 those rocks towards Ardchattan and the lower parts of 

 Loch Etive ; by which we are conducted, with little inter- 

 ruption in the recurrence of porphyritic and trap rocks, 

 first to Oban and the adjacent islands, and ultimately to 

 Inverary. Between those islands and Mull, the connexion 

 is not only geographically more intimate, but the simi- 

 larity more striking ; nor does there seem much reason to 

 doubt, that the trap of these islands and that of Mull 

 are all parts of a mass which, if it were not once connected 

 where a sea now divides it, was at least formed under 

 similar circumstances. That the various disjoined masses 

 of trap have actually been once continuous, is rendered 

 highly probable by the circumstance pointed out in Staffa, 

 which need not here be repeated; since the interval in 

 that case is not greater than in many others where no 

 such proofs of an original connexion exist. 



With respect to the porphyries of Argyllshire however, 

 it may be doubted if there is any reason for imagining 

 an identity in time between them and the proper trap 

 rocks. While the latter are almost invariably connected 

 with the secondary strata, the former lie on the primary. 

 Independent of this, and notwithstanding a community 

 of structure and relation to the rocks on which they 

 severally repose, and with which they interfere, those 

 porphyries present mineral characters sufficiently distinct 

 perhaps to justify a separate origin ; while their affinity 



