110 THE STRUCTURE OF THE [Ch. VI. 



end of this island, however, all order disappears, and the 

 masses are disposed in a dislocated manner. The direction 

 where it can be traced is north-easterly, and the dip towards 

 the east ; as is pretty generally the case in these islands. The 

 angle of the dip is various, often reaching to 70 or even to 

 80, and seldom declining so low as 1 5. In Barra, also, the 

 beds of gneiss can rarely be traced for more than a few yards 

 together in a straight line : they are commonly bent, and 

 even much contorted, while their inclinations to the horizon 

 are very irregular. In East Rona, as already noticed, the 

 gneiss often dips in opposite directions. In short, in these 

 islands, as on the mainland, the gneiss sometimes cannot be 

 detected in the form of beds ; and when the beds are apparent, 

 they exhibit all the complication of direction and dip which 

 might be expected, from their being curved on the great scale 

 in two lines, which nearly cross each other at right angles, 

 besides the lesser and subordinate curves, to which the larger 

 ones are continually subjected. To this structure of the 

 primary schistose rocks, assisted by the action of the ele- 

 ments, may be attributed the parallel arrangement of these 

 ranges of islands, and the subdivision of these ranges into the 

 individual isles. The strata in- the southern clusters are, 

 indeed, more regularly disposed ; but still, on the great scale, 

 they present the same characters : their composition is also 

 different, consisting in a great measure of clay-slate which, 

 at a distance from granite, is commonly arranged with a 

 greater degree of regularity than gneiss. 



Thus Macculloch* informs us, that the prevailing directions 

 in the shores of these islands will be found to correspond in 

 a great degree with the direction of the strata of which they 

 are composed, and to vary in a great measure according to 

 the changes in the lines of bearings ; and a similar general 

 correspondence will be discovered on the mainland between 

 the direction of the strata and the general forms of the 



* Western Islands of Scotland, vol. ii. p. 298. 



