36 STKATIGKAPHICAL GEOLOGY 



B. BRITISH KEPRESENTATIVES 

 1. Scotland 



By looking at any geological map of Scotland the reader will 

 see that, if the Outer Hebrides are included, about two-thirds of 

 the whole country are occupied by gneissic, schistose, and semi- 

 crystalline rocks, which are believed to be older than the Cambrian. 

 There is now no doubt that this great region of the Scottish 

 Highlands includes several series of pre-Cambrian rocks, and the 

 region is divisible into three areas : (1) a western area, including 

 the Hebrides, the western parts of Sutherland and Ross, and tracts 

 in. Skye, Mull, and Islay ; (2) a broad medial area extending from 

 the north coast of Sutherland to the south of Inverness ; (3) the 

 Central Highlands from Aberdeen to Argyllshire. 



Parts of these three areas are shown on the accompanying map 

 (Fig. 1), and they appear to include three different series of Archaean 

 rocks, though each area contains representatives of more than one 

 division, and it is only in the west that the relative positions of two 

 of these divisions are quite clear. 



The western belt is marked off from the median tract by a 

 thrust-plane of enormous displacement which is known as the 

 Moine thrust. The western outcrop of this plane runs from 

 Whiten Head near Loch Eriboll in a S.S.W. direction, through 

 Sutherland and Ross and through the Sound of Sleat. Thence it 

 probably passes outside the Isles of Mull and Islay. West of this 

 line the Archaean rocks are comparatively unaltered and seem, 

 indeed, to have undergone little change from the condition in which 

 they existed when the early Cambrian sandstones were deposited, 

 while the areas to the east of them have been subjected to enormous 

 pressures, with resulting dynamic metamorphism produced at some 

 post-Cambrian date. 



The north-west area in Sutherland and Ross has long been 

 known for its clear presentation of three distinct rock - systems, 

 though the precise ages of these three systems was for a long time 

 in doubt. Each is separated from that below by a great unconformity, 

 and the highest of them is now known to be Cambrian. The 

 other two have been named the Torridonian or Torridon sandstone, 

 and lowest of all the Hebridean or Lewisian gneiss. 



The following description of the Hebridean gneiss is taken from 

 the Report of the geological mapping of the area, published in 1888. 2 

 In the cliffs between Cape Wrath and Loch Torridon they present 

 two types of structure, appearing in some parts as " massive rudely 

 foliated crystalline rocks, with few divisional planes, or [in other 



