THE FRESH- WATER LOCHS OF SCOTLAND. 179 



certain that they possessed this banding and were thrown into gentle 

 folds before the intrusion of the later dykes. 



On referring to the map showing the surface geology of the Assynt 

 district, it will be seen that the Archaean area is traversed by narrow 

 dykes of igneous material (B G on map) trending west-north-west or 

 north-west. In certain belts they occur in great numbers, and their 

 intrusive character is clearly displayed. The dominant types in 

 the Assynt district comprise ultrabasic rocks (peridotite) and basic, 

 including dolerite and epidiorite. These dykes frequently form pro- 

 minent features in the landscape, sometimes giving rise to ridges and 

 sometimes to clefts or " slacks " in the midst of the surrounding gneiss. 



A further important feature in the history of the Archaean gneiss 

 remains to be noticed, for, after the uprise of the great series of 

 intrusive dykes, the whole region was subjected to mechanical stresses 

 that profoundly affected the pyroxenic gneisses and the dykes which 

 traverse them. These lines of movement may be described as lines of 

 shearing or disruption lines, which trend in certain definite directions, 

 and give rise to molecular re-arrangement of the minerals and the 

 development of newer foliation both in the gneiss and in the dykes. 

 The gneisses are thrown into sharp folds, and are traversed by zones or 

 belts of secondary shearing, in which the pyroxenic rocks are converted 

 into bio tit e and hornblende gneisses. In like manner, the basic and 

 ultrabasic dykes appear frequently as phacoidal masses in the shear 

 zones, and where the latter coincide more or less with the original 

 trend of the dykes, or cross them, then the peridotite and epidiorite 

 intrusions are changed into talcose schist and hornblende schist 

 respectively. A glance at the Geological Survey 1-inch maps of the 

 Assynt district (Sheets 107 and 101) shows the great number of these 

 lines of movement. Further reference will be made to these features 

 in connection with the rock-basins of that district. At present it is 

 important to remember that all these movements took place before the 

 deposition of the Torridmi Sandstone. 



This undulating plateau of Archaean gneiss was originally covered 

 by a vast pile of sandstones, conglomerates, and shales (Torridonian, 

 t on map), which has been largely removed by denudation. The 

 unconformability at the base of the Torridon Sandstone represents a 

 vast interval of time, during which the old land-surface of Archaean 

 gneiss was carved into hill and valley. On the north-west slope of 

 Quinag a remnant of this ancient topography is still to be found, where 

 a hill of crystalline gneiss rises to a height of 800 feet in the overlying 

 sandstone. One of the striking features in the landscape of that 

 region is the great western escarpment of Torridon Sandstone, reaching 

 in places an elevation of 1000 feet above the Archaean plateau. That 

 cliff is not continuous, for the sandstones on Quinag north of Loch 

 Assynt cannot be traced without a break to those of Cul Mor and Cul 



