856 J. GEXKIE ON THE GLACIAL PHENOMENA 



neighbourhood, the hearings of which vary from 1SLN.W. to W. 35° N. 

 On the slopes of Beinn Heabhal, at about 900 feet above the sea, I 

 found them pointing N. 35° "W., and the same direction occurs on 

 the ridge between Castle Bay and Tangasdal on the west coast. 

 But higher up the hill to the right the direction is "W. 35° N., and 

 this is also the trend on the high hill (Tangbhal) that forms the 

 south-west corner of Barra. 



That the contour of the island must have been, in preglacial 

 times, much more rugged and broken is well seen in the bevelled or 

 smoothed-off appearance which the edges of certain cliffs on the 

 southern slopes of Beinn Heabhal present. This appearance is very 

 conspicuous from the north shore of Castle Bay. 



The hills overlooking Borgh from the north are finely glaciated, 

 the striae bearing W. 35° N. to W. 40° K. A mile or so further to 

 the north they point W. 5° N". The road leading across the island 

 by Loch an Dun is overlooked by highly glaciated hills. A number 

 of striae met with in the hollow through which this road runs 

 pointed W. 10° N. to W. 12° K 



Barra is traversed by several hollows from shore to shore in a 

 direction that coincides with the trend of glaciation. This coinci- 

 dence, however, I am inclined to think is merely accidental, although 

 the hollows have, no doubt, experienced considerable glacial erosion. 

 They probably occupy lines of weakness, and owe their development, 

 in the first place, to ordinary atmospheric and aqueous erosion in 

 preglacial times. In some cases they are excavated along the line 

 of basalt-dykes, which, as a rule, in these islands weather much more 

 rapidly than the gneiss which they intersect. In other cases they 

 may lie in faults, while some certainly coincide with the strike of 

 the gneiss. In addition to these hollows, in which streams and 

 brooks are now constantly or intermittently flowing, there is a 

 minor set of dry hollows that cross the line of strike nearly at right 

 angles, namely from south-west to north-east. I noticed these only 

 in the southern part of the island, and they are best seen when the 

 ground is viewed from a little distance. I could not satisfy myself 

 as to their origin. They can hardly be faults, but may possibly 

 represent great joints. When we reflect upon the enormous time 

 these rocks of the Outer Hebrides have been exposed to subaerial 

 denudation (they probably represent the oldest land-surface in 

 Britain), we cannot be surprised to find structural peculiarities so 

 strongly brought out. The valleys and hollows that coincide with 

 the trend of the glaciation are, as a rule, broad and deep, and 

 generally smooth and continuous, having evidently been greatly 

 modified by the grind of the ice. Those hollows, on the other hand, 

 that do not follow the trend of the ice are usually, but not always, 

 narrow, shallow, irregular, and interrupted. 



4. Till or Boulder-clay. — The till is, as usual, of a dark or dull 

 bluish grey or greyish blue, sometimes paler, and now and again it 

 shows a brownish tint. It is seldom a true clay, but, as a rule, 

 merely a gritty earth, well packed and rammed so as to become 

 tough and unyielding. Finely divided argillaceous matter is usually 



