304 GENERAL COMPARISON OF 



and I need scarcely say that, as a preliminary to this 

 inquiry, it is here understood, with many geologists, that 

 the present strata, however at present placed, have 

 been originally deposited in succession, the one above 

 the other. 



Wherever strata follow each other with an inclination 

 directed to the same point of the compass, it is easy to 

 suppose that the lowermost in present position have always 

 occupied that place. But where, as in the case of these 

 islands, the dip is in any instance reversed, the deter- 

 mination is not so easy. There are two varieties of the 

 simplest case, that of a single reversal. If the strata 

 diverge from any imaginary centre below the surface, 

 on each side of the vertical, it must be concluded that the 

 lowest are to be sought among the outer, and that 

 the change of position has been produced by the subsi- 

 dence of the middle and vertical strata, or by two ele- 

 vations of the exterior and less inclined.* If, on the 

 contrary, the outer strata lie in the reverse order, conver- 

 ging toward the vertical from numerous points beneath the 

 surface, it follows that the middle strata are the lowest, 

 and that the change of position has resulted from an eleva- 

 tion in the centre, or two depressions at the extremities 

 of the series. *t In such cases there should be a corre- 

 spondence of the beds on each side of the centre, as far 

 as such correspondence is consistent with the general 

 limited continuity of strata, either in thickness or in com- 

 position. Where more changes than one occur in the 

 dip, the determination of the lowest becomes propor- 

 tionally more difficult. In the case of these islands, at least, 

 there is but one reversal to account for, and it belongs 

 to the first variety, that of divergence from a point beneath 

 the surface ; although many partial changes of that nature 

 occur among the continental strata adjoining. In Gigha 

 therefore, the lowest should on general principles be found 



* Plate XXXII. fig. 4. f Plate XXXII. fig. 5. 



