ST. KILDA. GEOLOGY. 57 



station on the overhanging precipices, they appear to 

 join each other in an obliquely vertical direction. The 

 deep covering of grass and soil on the surface, prevents 

 that junction from being seen where it would otherwise 

 be accessible. The general boundary of both can how- 

 ever be partially traced, by means of single rocks protru- 

 ding at the surface, and by the distinct nature of the 

 fragments which occupy approximate regions on the 

 slopes of the hills. This line lies between the north- 

 west and south-east points, leaving the town and the 

 hill of Conochan on one side, and nearly intersecting the 

 middle of the bay. The syenite is on the east side of 

 this line and the trap on the west. 



Where this boundary lies, numerous fragments of trap 

 penetrated by veins of syenite are found : these have 

 probably been detached from the line of junction, and 

 they present the only actual contact between the two 

 substances which admits of a close examination. 



It will be recollected that both in Mull and Rum there 

 is distinct evidence of the transition of the trap and the 

 syenite into each other, proving that they belong to a com- 

 mon deposit. There is probably the same community of 

 origin in St. Kilda ; however difficult it may be to account 

 for the complete and somewhat sudden change of character 

 between these two portions. It will indeed appear, that 

 in the cases which occur in these islands, where syenite 

 and trap exist together, three out of the four present 

 indications of a common origin ; and that nothing to 

 contradict that rule is discovered in Sky. It is probable 

 that it will be found to hold there also, and that the 

 examples will be discovered in those parts of the moun- 

 tainous region which are so difficult of access. The 

 appearance of veins passing from the syenite into the 

 trap, is the only circumstance in which the junction of 

 St. Kilda differs from those formerly described, and it 

 might seem at first view to afford an argument in support 

 of the posteriority of the syenite. But it will I believe 



