74 GENERAL COMPARISON OF 



of this deposit, which thus terminates at the place just 

 mentioned, by a thin edge. In examining the other 

 margins of this mass of strata where they are actually 

 visible, the superior beds are found in contact with the 

 primary rocks, indicating, in those places *also, the natural 

 terminations of the secondary rocks, although in a manner 

 less consonant to the ordinary appearances. The very 

 irregular form of this deposit is visible in the peculiar 

 position of the primary strata of Sleat with respect to the 

 whole ; but these inferences will more readily be deduced 

 from the general Map than from any verbal description. 

 In most places however, the borders of the strata are invi- 

 sible; either in consequence of the sea which conceals 

 them, or the trap by which they are covered or obscured ; 

 while many portions also have doubtless been removed by 

 those posterior changes so often alluded to. 



Notwithstanding these obstacles, -there appears reason to 

 conclude that the secondary strata of the Trap islands 

 form one deposit; analogous to those occurring elsewhere, 

 but separated, even from the nearest in Scotland, by an 

 interval so great as not to permit us to conclude that they 

 ever were connected, without assuming revolutions of the 

 surface greater than could be inferred from any evidence 

 we possess on this subject. 



One singular circumstance respecting these strata is 

 yet deserving of notice, and is deducible from the struc- 

 ture of Inch Kenneth ; the importance of which small 

 spot becomes visible in this general comparison : it was 

 not noticed in describing that island because its conse- 

 quences could not then have been foreseen by the reader. 

 Although, for the purpose of illustrating Mull, a rigid 

 examination both of Morven and Airdnamurchan was 

 made, no red sandstone was discovered in those districts, 

 nor did it occur in any part of Mull. On the contrary, 

 W'herever the secondary strata are accessible in the former 

 part of the mainland, the upper members, consisting of 

 the limestone and white sandstone, are in contact with the 



