200 FLANNAN ISLES. GEOLOGY. 



THE Flannan isles are all composed of gneiss, traversed 

 by numerous granite veins of different sizes and ramified in 

 all directions. Although the various cliffs of the Long Island 

 might be expected to show the disposition of the rocks 

 perfectly, some obscurity every where attends them ; from 

 the lodgment of plants, the growth of lichens, or other 

 causes. Here, every thing appears as if it had been cut 

 and polished by a lapidary ; and the consequence is, the 

 most accurate and minute detail of every contortion, bend- 

 ing, or displacement of the gneiss ; with a view of the 

 minutest ramification of the granite veins, and of the 

 disturbances which are communicated by them to the 

 main body of the rock. Those who have only examined 

 gneiss in the small portions in which it is commonly 

 accessible in mountains, and where, in general, little but 

 the surfaces of the beds are visible, will here perceive the 

 causes from which the confusion so common in this rock 

 arises.* They will also have little difficulty in tracing 

 it to the granite veins ; to the magnitude, position, and 

 ramification of which, the distortions of the gneiss will 

 always be found proportioned. 



These veins exhibit considerable varieties of compo-r 

 sition. Occasionally they are of a most minute grain, 

 often indeed so compacted as to resemble some varieties 

 of trap; in some places they have the aspect of an 

 ordinary massive granite, while in others they are formed 

 of large concretions, the most frequent character of the 

 granite veins which occur in the gneiss of this country. 

 Beautiful specimens of nacreous white felspar of great 

 magnitude are found among them, often accompanied by 

 similarly large crystals of black mica. In one place a 



* A diagram which consists of a curved line cut by two parallel 

 straight lines (PI. XII. fig. C.) will explain this appearance, the waving lipe 

 representing the bending of the beds of gneiss, and the straight included 

 portion that which is accessible at the surface. That portion which lies 

 above the uppermost line must be conceived absent, as that which is 

 below the lowermost is invisible. 



