202 FLANNAN ISLES. GEOLOGY. 



imprinted to mark its former situation. I may conclude 

 by remarking, that the granite veins of the Flannan isles 

 display precisely the same evidence of their posteriority 

 to the gneiss as is presented in the case of limestone or 

 schist by the similar veins that traverse these rocks. The 

 trap veins which pass indiscriminately through every sub- 

 stance that lies in their way do not furnish stronger proofs 

 of an origin more recent than that of the including strata. 



The gneiss of these islands is in general composed of 

 quartz, felspar, and mica; the latter ingredient, which 

 is rare in the Long Island, being here abundant. It 

 presents, as usual, many varieties, and among the rest 

 there is one which cannot be distinguished from common 

 granite. This consists of an equal mixture of flesh coloured 

 felspar and quartz, with but little mica. It forms beds 

 among the rest of the rock ; but as they lie in the most pre j 

 cipitous and highest part of the cliff their dimensions could 

 not be ascertained : some loose blocks measured three feet 

 in thickness. There is not a trace of the laminar tendency 

 in their structure, and therefore, although they may be con- 

 sidered as forming portions of the gneiss beds, they pre- 

 sent, in strictness, an example of bedded granite. Similar 

 examples have been already noticed on various preceding 

 occasions, but none before these exceeding a few inches 

 in thickness ; and all appearing to be parallel veins. The 

 circumstance is in this view important, as it may throw 

 light on the much disputed question of stratified granite. 

 If granitic portions of gneiss beds can vary in dimensions 

 from inches to feet, there appears no reason why they 

 should not attain a much greater size. Such rocks would 

 be termed stratified granite, if the circumstances under 

 which they appeared were such as to prevent a sight 

 of their real connexions. The limits which separate 

 granite from gneiss are indeed already sufficiently eva- 

 nescent, and of this perpetual gradation, scarcely a mile 

 of this survey does not offer an example. Yet the 

 geological distinctions of the two are, when considered 



