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 Archaean 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 the 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 Stouechrubie. 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 a 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 



