72 GENERAL COMPARISON OF 



division of the Trap islands. Considering the distance 

 between these two portions of the group, it might possibly 

 be doubted whether the secondary strata of Sky and the 

 adjoining islands do not form a separate deposit ; yet the 

 circumstances of correspondence are of such a nature as 

 to leave no reasonable doubts on that subject. 



Although the geographical connexion among the several 

 parts of the whole group is not intimate, it appears suf- 

 ficient, with the assistance of the adjoining portions of 

 the mainland, to admit of the geological approximation of 

 the different islands. 



The general position and the great extent of the gneiss, 

 renders it probable, in the first place, that the whole are 

 fundamentally connected, partly by means of this rock, 

 and partly by the primary sandstone which accompanies 

 and alternates with it. The last portions of the latter 

 strata to the southward, occur in Rum ; as, on the main- 

 land, they reach no further than Glen Elg. This sand- 

 stone, as will shortly be seen in the description of the 

 Sandstone islands, and as may be deduced from the 

 general map, is in a great measure limited to a certain 

 space on the north-west coast, its north-westernmost point 

 being found in Lewis, as the southernmost is in Rum : it 

 cannot therefore be known whether Egg and Muck repose 

 on it or on the gneiss : from the prolongations of the 

 continental strata they may be rather conjectured to lie on 

 the latter rock. 



With respect to the secondary strata, it has appeared 

 that they are found on the shores of Morven and Airdna- 

 murchan, as well as in many of the islands under review. 

 The connexion between those two tracts is sufficiently 

 obvious in the general agreement of all their rocks ; while 

 the identity of the secondary strata of Mull with those of 

 the former is perfect. The resemblance between the cal- 

 careous strata of Airdnamurchan and of Muck is also such, 

 as, together with their proximity, to justify the deduction 

 that they are parts of the same general deposit. By this 



