566 HISTORY OF THE OUTER HEBRIDES. 



other islets in the Firth of Forth on the east, but the 

 " plugs " up which the molten matter flowed in greater 

 quantities than it did where the gneiss in remoter parts 

 cracked into chasms which became moulds for the liquid 

 rock to fill. The Scuir of Eigg seems to be of later age. 

 There we have the Cast standing out as a gigantic fragment 

 of a wall after the mould had weathered away. Accord- 

 ing to Geikie the mould was formed by a river canon or 

 gorge running westwards. The Hysker rocks according 

 to the late Professor Heddle appears to be a continuation 

 of this " Cast " sloping westwards towards the Atlantic. 



THE BOULDER-CLAY. 



It is thought to be somewhat singular that no vestiges 

 of the rock fragments from the mainland or from the 

 neighbouring island of Skye have been found in the 

 boulder-clay that so thickly covers the Outer Islands and 

 forms the sub-soil of them. This is deemed singular only 

 to those who insist on the ice-sheet or other glaciation 

 agency coming from the east across the Minch and from 

 the mainland. Certainly, if it did so, then it must have swept 

 up portions of the rocks of Skye, but none such are found 

 in Lewis at any place. The fragments are from rocks 

 which have their parent source in the island itself. A 

 glance at the levels of the Lewis will shew that the surface 

 slopes by gentle gradients to the north from their base at 

 the Harris hills. At the foot of the hills in the Park 

 district morain heaps abound pointing to local glaciers 

 which slowly glided down their northern slopes. The 

 accumulation of boulder-clay is greater as we go north 

 towards Ness where it is at its greatest. The longer axes 

 of the multitudinous sheets of water from those of the 

 size of Loch Langabhat in Uig are on the whole from 

 south to north. All these features point to the direction 

 in which the clay-laden agency travelled. 



During the Glacial Age, when these Outer Hebrides 

 in common with all Scotland lay buried under the vertical 

 pressure of thousands of feet of snow and ice, this vertical 



