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^ 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 biotite 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 Torridon 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 
