OF THE OTJTEE HEBKIDES. 



855 



black or very dark blue rock, and in some places, especially in the 

 thicker parts of the veins, it is finely vesicular at its junction with 

 the gneiss. In the broader dykes the rock is often vesicular in the 

 centre, a common appearance in the basalt-dykes of the Lowlands. 

 The accompanying sketch (Fig. 11) is a ground-plan of a surface 

 of gneiss, showing some characteristic forms assumed by the veins. 



Fig. 11. — Ground-plan of basalt-veins intersecting gneiss, near North 

 Bay, Island of Barra. (Thickness of veins shown in inches.) 



3. Glaciation. — Like all the other islands described, Barra is 

 abundantly glaciated, the average direction of the striae being about 

 W. 35° N. The hills have the usual abraded and rounded aspect, 

 and are distinctly smoothed off from south-east. But striae are not 

 so well preserved as a rule as in South IJist. The flanks of Beinn 

 Eribhal in the north, which are highly glaciated, only showed striae 

 in two or three places, which pointed W. 30° N". The rock here is 

 a coarse-grained hornblendic gneiss, penetrated by granite-veins 

 and weathering rapidly. Along the east side of the island striae 

 were noted in a number of places, all of them agreeing in direction, 

 W. 30° N. to W. 35° N. At Castle Bay striae occur close to the 

 sea-shore, the bearing given by Mr. Campbell being N.N.W. true. 

 But there are a good many other striated faces in the immediate 



