58 ST. KILDA. GEOLOGY. 



be found, that these are not real veins. A similar appear- 

 ance occurs in Arran, and in circumstances which pre- 

 clude all possibility of its arising from the ramification 

 of one rock into another. It seems a variety of that 

 concretionary structure which is known to occur under 

 other forms, in this rock as well as in granite. 



As the state of the weather did not permit me to 

 approach near to Borera, I can only conjecture, from its 

 form and colour, that it consists entirely of trap, as I 

 observed no indication of the peculiar disposition which 

 attends the syenite. 



Such is the mineral history of St. Kilda, solitary in 

 structure as it is in position ; since the nearest rocks on 

 the east side consist of gneiss, and the first we meet with 

 on the west, Rockall, is formed of granite. 



