LAKES IN RELATION TO GEOLOGICAL FEATURES 461 
Orkney, thence westwards along the strike of the Old Red Sandstone 
strata by the north coast of Sutherland. The tributaries of this 
river in some cases followed the pre-Old Red Sandstone vallevs in 
the old floor of crystalline schists, filled with the basement rocks of 
that formation. As these ancient valleys coincide with the strike 
of the schists, the streams, by cutting backwards along the grain of 
the rocks, have been enabled to intercept a great part of the head- 
waters of the original south-east consequent streams and deflect them 
towards the north. The River Naver is an excellent example, for it 
has succeeded in capturing the upper portion of the Helmsdale 
consequent stream from below Loch Naver to the sources of that 
river. 
Reference has already been made to the structural evidence 
suggesting that the subsequent or longitudinal rivers draining into 
the Moray Firth intercepted the old consequent streams of the 
Northern Block as far west as Loch Eil. While this was in progress, 
however, the weaker mesozoic strata overlying the palaeozoic and 
schistose rocks in the west were also being removed by rivers cutting 
back from the Atlantic Rise, and forming the plain of the Minch, 
during the period of the production of the Continental Shelf Another 
drainage system seems to have followed the line of the North Channel 
and reached the shatter belt of the Great Glen somewhere about the 
Firth of Lorne. Along this line of weakness it seems to have cut 
backwards, intercepting in turn the consequent streams of the 
Northern Block. The order in which the streams were appropriated 
is probably shown by the decreasing depth of the valleys from west 
to east. The Sound of Mull, Loch Eil, and Loch Arkaig represent 
depressions initiated by the old consequent streams of the Northern 
Block. 
The prominent physical features in the Outer Hebrides are due to 
a ridge of Lewisian Gneiss which has been isolated by the denudation 
of the western rivers draining to the edge of the Continental Shelf. 
Skye, which originally formed part of the mainland, has been dis- 
connected by similar processes of denudation. The mountainous 
tracts of that island, of Rum, and Mull are due partly to the resistant 
nature of the Tertiary plutonic masses and associated volcanic rocks, 
and partly to the influence of faults which have brought up portions 
of the old floor of crystalline schists and overlving Torridon Sand- 
stone. 
As already indicated, the lOO-fathom line was the base-level of 
erosion at the period of maximum elevation. Inwards, however, the 
plain now represented by the Minch acted as the base-level of the 
obsequent streams. Owing to their steeper gradient, the rivers 
flowing west ultimately developed flat-bottomed valleys, and, as they 
