56 Dr. Mac Culloch's Sketch of the 



tlie mode of its formation. The other I have called mountain 

 trap. 



The terraced trap forms by far the greater portion of the surface 

 of Sky ; but as it is sufficiently defined in the map and in the ge- 

 neral description, I will forbear here to name its boundaries. Its 

 .northern and principal tract is every where continuous, but at the 

 southern side of the island there are detached portions, which I 

 shall first notice. The southernmost in position is a hill of no 

 great extent, which is seen above the secondary strata already de- 

 scribed at Swenish point, between Loch Slapin and Eishort. This 

 mass is connected with two large bodies like roots, (inasmuch as 

 they have neither the parallelism nor the independence of trap 

 veins) which cut through the whole mass of strata, and disappear 

 below the sea. I consider this place as of great importance in the 

 history of trap, as it shows plainly how a particular mass of that 

 rock may appear fairly incumbent on a given stratum, while it is in 

 fact connected with a much deeper set of rocks. 



It is plain that partial views of this mass at any one point of all 

 the various substances which it traverses, would assign to the same 

 rock all the several hypothetical characters according to which trap 

 has sometimes been divided. It would be called in one place primi- 

 tive, in another transition, in a third floetz, and so forth. As I do 

 not intend to enter into the well-known questions respecting the 

 origin of trap, I forbear to point out how this appearance bears 

 on any of the hypotheses which regard its formation. 



The next of the stratified masses of trap which are seen toward 

 the south are the hills w^hich decline from Blaven to Strathaird, and 

 appear to be similarly incumbent en the stratified rocks of that 

 promontory, but I had no success in my attempts to discover their 



