THE FRESH-WATER LOCHS OF SCOTLAND. 
187 
area cannot fail to be struck with the number and irregular outlines 
of the lakes in the plateau of Archsean gneiss. While mapping that 
region, it was obvious that the direction of many of the lochs and of 
their branches had been largely influenced by the trend of lines of 
shearing and lines of fault, by the trend of groups of intrusive dykes, 
and by the presence of ultrabasic masses, which weather more readily 
than the pyroxenic gneiss. In view of these facts, the irregular contour 
of the lakes on the Archaean plateau, as proved by the soundings, is 
what might naturally be expected. 
Loch Assynt , — This is th6 largest and by far the most important 
lake in the Assynt district. Round the upper end and along the 
north-east shore from Inchnadamph to the southern base of Quinag, 
it is floored by Cambrian and Torridonian strata, while the remainder 
rests on the Archaean gneiss plateau. It lies along an old consequent 
valley, the origin of which dates back to a time when the surface 
configuration was very different from what it is now. Originally, the 
lake was of larger dimensions, for at its upper end it has been silted up 
by the river Loanan; indeed, in that direction it must have extended 
at one time almost to Stonechrubie. At its lower end it must formerly 
have continued down to the narrows above Inveruplan — a distance of 
over two miles from the foot of the loch, where a rocky barrier of gneiss 
and intrusive dykes crosses the river Inver. From that point upwards 
to the present lower limit of the lake an alluvial terrace is traceable, 
through which the river follows a winding course. During its former 
extension. Loch Assynt must have been continuous with Loch Uidh na 
Geadaig and Loch Leitir Easaich. 
The soundings show that this rock-basin is comparatively uniform. 
The 50-feet contour-line runs from the present lower limit of the lake 
to near the mouth of the river Loanan ; the 100-feet contour-line, from 
the bend at Loch Leitir Easaich to near the schoolhouse at Inchna- 
damph- — 3, distance of 5 miles; the 150-feet contour-line is continuous 
from a point opposite Tomore to near the schoolhouse at Inchnadamph, 
thus forming one basin 4| miles long. Five basins are enclosed by 
the 200-feet contour-line, and three basins by the 250-feet line. The 
height of the surface of the lake above sea-level is 215 feet, and the 
greatest depth is 282 feet, within the Archaean area near Tobeg and 
Eilean Assynt. At that point the lake is 67 feet below sea-level. A 
glance at the bathymetrical map will show that the long axes of the 
deeper basins coincide with the trend of the loch between Loch Leitir 
Easaich and Inchnadamph, and that they lie nearer the southern shore. 
This feature is worthy of note, as it is a continuation of an important 
fault which has been traced for miles along Glen Salach in a north- 
west direction, in the line of which lie several lakes (see Geological 
Map). It must be borne in mind, however, that this line of disruption, 
which has produced brecciation of the Archaean gneiss and dykes along 
