THE SCHISTOSE ISLES. 305 



on the eastern side, and in all the rest of these islands on 

 the western. For if it be supposed that the whole was 

 originally a single series of flat beds, it will appear that 

 the present disposition was the result of a depression 

 between the two portions that constitute Gigha and Isla, 

 or else of two equivalent elevations at the extremities. 



Besides the mode now mentioned, there are two others 

 in which the question of superiority may in certain cases 

 find its solution ; the presence of granite in ascertaining 

 the lowest, and that of the secondary strata in determining 

 the uppermost rocks. But a little consideration will show 

 that both of these offer but imperfect, and often very 

 partial solutions of the difficulty. In the case of granite 

 it is often found that the upper members of a series are in 

 contact with it ; a natural result of the irregularity of its 

 surface and the obliquity of its position to the stratified 

 rocks ; while, in that of the secondary strata, it is obvious 

 that they can afford no assistance unless where they follow 

 the primary in conformable order. But whatever the 

 value of these criteria may be, there can be no recourse to 

 them here. Granite is not found in these islands, nor 

 can any assistance be derived from the small portions of 

 secondary strata which are visible. These lie in an uncon- 

 formable manner on the clay slate or other primary rocks, 

 and are therefore incapable of pointing out that which 

 is uppermost. It is true that the same strata occur on 

 the mainland adjoining; but, as far as I have seen, the red 

 sandstone occupies there a similar relation to the primary 

 strata, and is therefore incapable of removing a difficulty 

 which must perhaps for ever remain unsolved, if the pre- 

 ceding hypothetical explanation is of no value. 



The impossibility of determining this point, puts it out 

 of our power to ascertain the original thickness of the 

 mass of strata which is found in these islands ; a circum- 

 stance, which, in other cases, for want of attention to the 

 facts just mentioned, and to others which appear suffi- 

 ciently obvious, seems to have been occasionally over- 



VOL. II. X 



