FLANNAN ISLES. GEOLOGY. 201 



vein of a complicated structure occurs, the middle lamina 

 being pure quartz terminating at each side in a fine grained 

 granite. The graphic variety also is here to be seen. 



While the difficulty of tracing the position of gneiss 

 on the surface has frequently caused geologists to enter- 

 tain doubts respecting its stratification, it has equally led 

 to misconceptions on the nature of its connexion with 

 the granite veins; if indeed some of the received opinions 

 on this subject have not rather been of a hypothetical 

 nature than founded on real observation. These veins have 

 been supposed integrant and original portions of the gneiss, 

 or of contemporaneous formation, and they have also been 

 said to originate and terminate in the gneiss. It is easy 

 to see that such misconceptions respecting the stratifi- 

 cation of this rock would follow from those partial views 

 which the diagram above given is meant to illustrate. 

 Here there are no independent parts, but it is quite easy 

 in imagination so to dispose of the several portions of 

 rock as to give that irregular and discontinuous appearance 

 which is generally seen in partial examinations of the 

 surface. 



In these islands the regular prolongation, continuity, 

 and ramification of the veins, is also perfectly apparent ; and 

 the eye traces them with the same facility as it does the 

 contortions of the gneiss. Another fact is here visible 

 which is also of importance in the history of these veins. 

 This is their distinctness from the rock which they traverse. 

 Most observers have, I believe, imagined an intimate con- 

 nexion, or amalgation, to exist between the veins and the 

 rock ; a deception arising from similarity of ingredients, and 

 from that equable hardness which causes both to yield in the 

 same degree to the hammer. The sea has here served to 

 detect that which the hammer can not. Wherever it has 

 had access (and its spray reaches almost every where) the 

 vein is perfectly distinguished from the rock ; and what- 

 ever its texture may be, it is separated without difficulty, 

 often falling out for a small space and leaving a furrow 



