440 THE FRESH-WATER LOCHS OF SCOTLAND 
rocks are specially developed, while granular hornblende rocks and 
biotite gneisses are characteristic of the northern and southern tracts. 
Sometimes these rocks appear like ordinary eruptive masses, some- 
times with crude mineral banding, and yet again with well-defined 
foliation. 
The schists of sedimentary origin have a limited development 
north of Loch Maree and near Gairloch. The prominent members of 
the series are quartz-schists, mica-schists, graphitic-schists, limestones, 
and dolomites, with tremolite, garnet, and epidote. They are there 
associated with a massive sill of epidiorite and hornblende-schist. 
After the development of the earlv mineral banding of the 
gneisses, the Fundamental Complex was pierced by a remarkable series 
of igneous intrusions in the form of dykes and sills ; comprising 
ultrabasic rocks (peridotite), basic rocks (dolerite and epidiorite), and 
acid rocks (granite and pegmatite). The evidence in the field points 
to the conclusion that the ultrabasic rocks cut the basic, and that the 
granite dykes and sills were intruded into the gneisses after the 
eruption of the basic dykes. 
After the intrusion of these various igneous materials, the whole 
region was subjected to terrestrial stresses which affected the gneisses 
of the Fundamental Complex and the dykes which traverse them. 
These lines of movement traverse the Lewisian plateau in various 
directions, producing planes of disruption, molecular rearrangement 
of the minerals, and the development of foliation in the gneiss and 
dykes. Li these zones of shearing the coarse pyroxenic gneisses are 
replaced by granulitic biotite and hornblende gneisses, and the basic 
dykes merge into bands of hornblende-schist. 
After the cessation of these terrestrial movements, and before the 
deposition of the sediments that now^ form the overlying Torridon 
Sandstone, the Lewisian Gneiss underwent prolonged denudation. In 
V the north-west of Sutherland, between Durness and Loch Laxford, the 
surface of these ancient rocks was worn down to a comparatively level 
plane ; but farther south, in Assynt and onwards to Loch Torridon in 
Ross-shire, it was carved into a series of deep narrow valleys with 
mountains rising to a height of about 2000 feet. 
TORRIDONIAN 
Throughout the North- West Highlands the Torridon Sandstone 
rests on the various members of the Lew^isian Gneiss with a violent 
unconformability, which must represent a vast lapse of time. This 
formation is divisible into three groups : a lower, composed of 
epidotic grits and conglomerates, dark and grey shales with calcareous 
bands, red sandstones and grits ; a middle, consisting of a great 
succession of false-bedded grits and sandstones ; an upper, comprising 
