THE CLYDE ISLANDS. 503 



to repose on the sandstone or other secondary strata in 

 the vicinity. 



It will also be remembered that Morven presents a very 

 remarkable instance of this association. Here, the trap 

 appears to lie on the gneiss, but is rarely or never in 

 actual contact with it ; a body, however thin, of secondary 

 strata being almost every where interposed. In Mull also 

 it must have been observed, that the trap ceased at the 

 Ross where the gneiss became visible ; neither lying on 

 that rock nor on the granite, but on those parts only where 

 the existence of secondary rocks was either demonstrable 

 or might fairly be inferred. Airdriamurchan is similarly 

 constituted ; the limits of the trap in that promontory 

 appearing to be nearly, if not exactly, the same as those of 

 the secondary strata which are there found. It is unneces- 

 sary, in confirmation of this remark, to repeat that which 

 may be easily deduced from the descriptions of Sky or 

 of the other islands where the overlying rocks occur. 



Whatever may be judged of this circumstance of a 

 prevailing association between the trap rocks and the 

 secondary strata, it is by no means universal,, and must 

 not be esteemed a general rule. Even in Mull, the trap 

 of Gribon rests partly on the primary strata of that 

 shore. It has also been seen, in the account of the Slate 

 isles, that although the trap of that district lies in many 

 places on the secondary strata, it was, in others, in con- 

 tact with the primary schists ; in .which state it also 

 appears to exist in different parts of the adjoining main- 

 land. In the former remarks on this subject, the porphy- 

 ritic district of Glenco was pointed out as another excep- 

 tion; and that exception may be extended to the great 

 mass of porphyry found in the northern parts of Argyll- 

 shire, which appears also to rest on the primary rocks. 



In the article already alluded to, it was suggested that 

 this phenomenon was difficult of explanation. Perhaps 

 no real difficulty exists, and no necessary connexion 

 between the two, even to the limited extent now apparent, 



