1873.] GEIKIE — GLACIAL PHENOMENA OP OUTER HEBRIDES. 543 



as I could judge, are of any depth. The more strongly pronounced 

 lakes frequently rest at the foot of well-marked escarpments, and 

 sometimes in broad hollows between long parallel banks of gneiss. 

 Their shores are frequently mossy all round ; again they are often 

 skirted by till and rock ; but I saw no true rock-basins among them. 



The trend of these lakes is obviously due to the structure of the 

 ground, and has nothing to do with encroachments of peat. They 

 coincide in direction precisely with the strike of the gneiss ; and 

 when any decided variation in the strike of the gneiss takes place 

 there is a similar change in the direction of this group of lakes. 

 This is well seen immediately to the west of Stornoway, where the 

 lakelets are observed to follow the strike, which is at first east and 

 west, gradually wheeling round to south-west. 



The explanation of their origin appears to be simply this. The 

 land-ice in its progress over the island had to encounter a series of 

 low, rough, and interrupted escarpments. Flowing against the dip 

 of the gneiss the stream would tend to deposit till in front of the 

 cliffs, knolls, and rocky ridges that faced to the north-west. But 

 the deposition of this till would be very irregular ; and consequently, 

 when the ice finally melted away, the hollows between parallel 

 ridges of rock would be found unequally coated with ground-moraine. 

 There would thus be ready for occupation numerous hollows with a 

 prevalent north-east and south-west trend. As a matter of fact I 

 found that the ends of such lakes, when not obscured with peat, 

 were dammed up either wholly with till, or with till and rock 

 together. 



The manner in which the two groups of lakes now described fre- 

 quently unite offers no difficulty. In many places the old strike- 

 escarpments have been cut across by the ice at right angles ; and 

 thus a new system of ridges and hollows has resulted. Hence it is 

 not surprising to find that not only lakes but also streams exhibit 

 both directions — now trending north-west and south-east, and then 

 turning sharply off at right angles to the course previously fol- 

 lowed. 



Besides the two groups of lakes now described there are some 

 shallow sheets of water which, as already remarked, have no deter- 

 minate direction. But these we can hardly separate from the 

 others. They owe their origin either directly or indirectly to the 

 glaciation. A few, however, may lie in shallow depressions dammed 

 up by peat. 



c. Lalces of the Mountain Region. 



These, with some exceptions, arc all due to glacial erosion. But 

 it will be more convenient to give an account of them and the large 

 sea-lochs — Resort, Scaforth, Claidh, and Bhrollum — when I come 

 to treat of the glacial phenomena of Harris. 



VII. Conclusion. 



From the facts now advanced it is evident that Lewis has been 

 traversed from south-east to north-west across its whole breadth by 



2o2 



